GALAPAGOS RIFT LIMPET NEOMPHALUS 307 they are in close association with the vesti- any damage to the periostracum of the lower­ mentiferan Riftia pachyptila Jones (1981). most shell. The periostracum should provide Vent effluent at the Garden of Eden vent-field a seal along the shell edge that would protect has a maximum temperature of 17°C» in con­ it from the claws of the brachyuran crab trast to the ambient bottom temperature of Bythograea thermydron Williams (1980), a approximately 2°C, Vent effluent contains potential predator at the Galapagaos Rift. The hydrogen sulfide and is reported as anoxic foot of Neomphalus is sufficiently muscular for above 10°C» but presumably mixes sufficient­ locomotion. Some motility would be required ly with oxygenated ambient water to sustain for the mating we deduce from the anatomy the limpets. Current flows of 2 to 10 cm/sec (Fretter, Graham & McLean, 1981). have been measured (all data from Corliss et Suspended bacterial cells in the rift-vent ef­ al.» 1979, p. 1082). The limpets are often in fluent have been measured in the range of 5 contact and some are positioned on the shells x 105 to 106 per m| (Karl et aL» 1980) during of others, as shown on the large fragment of the January 1979 expedition; Corliss et al. pillow basalt from the Garden of Eden (Fig, (1979) reported a count of 108 to 103 bacterial 12A), The broad anterior surfaces of the cells per ml in preserved samples from the limpets on the boulder (Fig. 12A) are facing in 1977 expedition. Thus there is a sufficient different directions, indicating that there was source of suspended food to sustain large no orientation with reference to currents. populations of filter-feeding animals. Mats of Neomphalus may attach to the tubes of Riftia microorganisms also develop on shell or rock (Fig. 12B), although there is no indication of surfaces in the vicinity of the vents (Jannasch this in Fig. 12A. & Wirsen, 1981), providing a source of food Neomphalus is primarily sedentary; the for limpets that feed by grazing. shell margin is irregular, evidently conforming Gut contents in Neomphalus suggest that to a particular site. Those attached to other feeding is a combination of grazing and filter shells leave no attachment scars nor cause feeding (Fretter, Graham & McLean, 1981).

FIG. 11. Oyster Bed vent-field, dive 726, showing the vestimentiferan, Riftia pachyptila, the brachyuran crab Bythograea thermydron in upper center, the galatheid crab at lower left, and numerous Neomphalus fretterae on all exposed surfaces. 308 MCLEAN

FIG. 12. A) 72 lb fragment of pillow basalt from dive 733, Garden of Eden, photographed on deck of support ship, showing Neomphalus in place and tubes of the vestimentiferan, Riftia. B) Tube of Riftia with attached Neomphafus in place, from 1979 expeditions, dive number unknown.

Wear on the rachidian and lateral teeth (Fig. proximal arm of the genital duct, and that fer­ 2D) provides additional evidence that the tilized eggs receive a coating of jelly-like ma­ radula is used for grazing. The prominence of terial before extrusion from the distal arm of the jaw and buccal development and retarda­ the genital duct (Fretter, Graham & McLean, tion of the gill development in juvenile speci­ 1981). Egg capsules have not been collected; mens (Fig. 10D) suggests that grazing is the thus, the next step is unknown and it is un­ exclusive feeding mode of young stages. A certain whether individually encapsulated retention of the grazing capacity and a com­ eggs are released freely or attached to the bination of the two feeding modes in adults is substratum. A sufficient number of females therefore not surprising. have been collected to rule out the possibility Sectioned specimens examined by Fretter, that developing young are brooded under the Graham & McLean (1981) showed ripe shell. Egg masses have apparently not been gonads with gametes in all stages of develop­ found attached to the boulders from which the ment, indicating that reproduction is a constant specimens were collected. The free release of process throughout the year, in agreement coated eggs therefore seems most likely. with observations that in the absence of A coated egg, upon expulsion from the seasonal stimuli, most deep-sea invertebrates mantle cavity might settle in a crevice or per­ spawn throughout the year (Rokop, 1974; haps become entangled by the byssal threads Rex et aL, 1976). of the rift-vent mytilid. A postprotoconch larval The reproductive anatomy of Neomphalus shell with a sharp transition preceding the-on­ indicates that copulation must take place, that set of adult sculpture is lacking, indicating that sperm are stored in a receptaculum seminis, there is no planktotrophic veliger stage that fertilization probably takes place in the (Shuto, 1974; Robertson, 1976). Plankto- GALAPAGOS RIFT LIMPET NEOMPHALUS 309 trophic veligers are unknown in archaeo- ordinary combination of archaeogastropod gastropods (Fretter, 1969) and Neomphalus and mesogastropod characters combined is no exception. Direct development through with some unique features. That it is a highly the trochophore and veliger stages probably modified and specialized archaeogastropod takes place within the egg coating; crawling cannot be doubted, for it has such primitive juveniles would emerge. During the growth of archaeogastropod characters as a rhipido- the first and second postprotoconch whorls, glossate radula, a bipectinate ctenidium, the juvenile Neomphalus would be active but epipodial tentacles, and the anterior loop of would remain in crevices or among the byssal the intestine. Its features at the mesogastro­ thread of the mytilids. When the transforma­ pod level of organization include the nearly tion to the limpet is completed by the end of complete reduction of the right pallial com­ the second postprotoconch whorl, the limpets plex, a monotocardian circulatory system, would take up a more sedentary, primarily expansion of the left kidney and formation of a filter-feeding existence where exposed to the nephridial gland, a copulatory organ in the strong flow of the rift-vent effluent. Those male, and glandular gonoducts in both sexes. juvenile specimens received were recovered Unique features include the split osphradia, from residue samples associated with the absence of a snout, dorsal position of the food mussels. The mature mussels live in a zone groove, posteriorly directed cephalic tenta­ further away from the vents; thus there is cles, the enlargement of the left tentacle to some evidence that the early life of the juven­ form a copulatory organ, and an unusually po­ ile takes place away from the vents. sitioned receptaculum seminis in the female. The hypothesized course of development Fretter, Graham & McLean (1981) discuss should enable the continuation of populations the leftward rotation on the anterior-posterior at each vent site, but it does not account for a axis and the 90° of further torsion, so clearly mechanism of dispersal to more distant vent shown in the placement of the internal organs, sites. Individual vent fields have been postu­ that accounts for many of the unusual aspects lated to have a rather brief, ephemeral ex­ of the anatomy. These shifts and rotations istence of several hundred years, necessitat­ can be understood as resulting from the early ing the colonization of the new vent sites that ontogeny, as described here, in which growth emerge along the spreading sea floor. stops along the columella, forcing the colu- Unlike Neomphalus the mytilid from the mellar muscle to emerge to the base of the Galapagos Rift seems to have an effective shell, and changing the orientation of the ani­ dispersal mechanism. Because if has a well- mal from its initial axis of coiling. Can it be defined larval shell, Lutz et al. (1979) inferred shown that some of the features of this ontog­ that there is a planktotrophic larval stage eny occur in the evolutionary history of capable of long-range dispersal via bottom Neomphalus? Although Neomphalus fret- currents, its metamorphosis indefinitely de­ terae is the only known member of a group layed because of lower metabolic rates at that can be assigned to no family, superfamiy, ambient bottom temperatures. For Neom­ or suborder with living representatives, its phalus, however, the colonization of new evolutionary history can be sought in the fossil record, even though no fossil record of the vents may be a matter of passive transport via 3 larger, as yet unknown animals that may genus itself has been found. move between the springs. Argument for an Archaic Origin

DISCUSSION The neomphalid ctenidium is a departure from other gastropod ctenidia. It is a mor­ As discussed by Fretter, Graham & McLean phological innovation, an effective adaptation (1981), the neomphalid anatomy is an extra- for filter feeding. The course of evolution is

^Four poorly known Devonian genera, Procrucibulum, Paragalerus, Progalerus, and Protocalyptraea, have names that imply some similarity to the shell form of calyptraeids. An affinity of these genera to the Calyptraeidae, which appeared in the Cretaceous (Hoagland, 1977) has to be ruled out. However, these genera are of interest as possible precursors to the Neomphalidae. Except for Paragalerus, drawings of reconstructed shells were illustrated in the Treatise (Knight et al., 1960). Each genus is known only from the type-species (Yochelson, personal communication), holotypes of which were described and illustrated by Knight (1941). The first three are represented by internal molds that lack information about protoconchs and muscle scars. Protocalyptraea is based on a small incomplete specimen (see also Linsley etal., 1978:111), in which the peripheral frill would seem to preclude it as a precursor for Neomphalus. Affinity of these genera with the Neomphalidae cannot be completely dismissed, but it cannot be discussed further until better material is known. 310 McLEAN

marked by adaptive radiations, proliferations lies that it is apparent that the form itself im­ of new taxa following the introduction of suc­ poses few constraints upon the internal cessful morphological innovations (Simpson, anatomy. Thus, the major features of a lim­ 1953; Stanley, 1979). Thus, the neomphalid pet's anatomy must be a reflection of primitive ctenidium should either have given rise to ex­ characters in its coiled predecessor. perimentation or be an end result of experi­ In the absence of a living coiled group with mentation that has already taken place. Be­ anatomy comparable to that of a particular cause Neomphalus has many unique and limpet, one may hypothesize the anatomy of very specialized features and because it oc­ the coiled predecessor, basing the recon­ curs in an environment with many limiting struction around the characters displayed by parameters, it surely must represent a single the limpet that are assumed to be primitive twig of a larger branch in a group having the and not a consequence of the limpet mode. same ctenidial structure. Its predecessors Although the ctenidial filaments of Neom­ need not be limpets, for limpets are evolution­ phalus are highly modified for filter feeding, ary dead ends, giving rise to adaptive radia­ the basic configuration of the neomphalid gill tion within a family or superfamily, but not —aspidobranch with afferent attachment serving as raw material for the further evolu­ lacking—is a character that would be shared tion of higher categories. with the coiled predecessor. The only com­ The limpet form has been derived from parable condition in which an aspidobranch coiled predecessors with some frequency gill lacks an afferent membrane occurs in the in gastropods. Among archaeogastropods, Pleurotomariidae, in which the gills are mesogastropods, opisthobranchs, and pul- paired. The Pleurotomariidae are regarded as monates there are many families of limpets. the most primitive living gastropods. The One example is known in a siphonostomate superfamily Pleurotomariacea has a fossil neogastropod—that of Concholepas. Except record that is continuous from the Upper for the docoglossate patellaceans, for which a Cambrian. The possible affinity of Neomphal­ convincing derivation has never been offered, us to the extinct groups contemporary with the the limpet families are closely related to fami­ early pleurotomariaceans must be consid­ lies or superfamilies having regular coiling, ered. particularly those in which the shell aperture is Although the subordinal classification of holostomate rather than siphonostomate. archaeogastropods proposed by Cox & In some families or superfamilies—for ex­ Knight (1960) for use in the Treatise (Knight et ample the trochacean Stomatellidae—there al., 1960) is due for modification, all of the are limpet derivatives in which the entire pro­ major divisions they recognized are traceable gression from a trochiform to auriform and to a to the early Paleozoic, the only remaining limpet shell form is represented. In others, like doubt being that surrounding the appearance the Patellacea and the Calyptraeidae, there of the Patellina—whether early or late in the are no clues as to the shell form of the closest Paleozoic. Most of the living archaeogastopod relatives. In these groups the derivation may families made their appearance by the early have been sudden, in a process of paedo- Mesozoic, well in advance of the burst of evo­ morphosis, a phylogenetic derivation in which lution in the Neogastropoda during the reproductive maturity is attained in a stage Cretaceous. If all other high-level, subordinal before the development of adult characters origins and initial radiation of archaeogastro- (see Gould, 1968; Stanley, 1979). Normal pod taxa took place in the Paleozoic, it is logi­ adult coiling does not take place; rather, shell cal to assume that the subordinal distinction in growth expands the aperture of the juvenile Neomphalus also had a Paleozoic origin. shell. In each case the limpet's anatomy, Excluding the living and fossil groups for though modified by loss of coiling, retains a which there is reasonable certainty that the sufficient number of characters common to its gill condition was dibranchiate, and excluding ancestor (shared primitive characters) to the neritaceans, a completely divergent line permit its taxonomic placement. The external (Fretter, 1965), for which the fossil record is features of any limpet animal—for instance well understood, those extinct, conispirally the modifications of the head for its generally coiled archaeogastropods that may have had constant retention under the protective shield a unibranchiate mantle cavity were placed by of the shell—have some similarity from one Knight et al. (1960) in two of the suborders of family to another, but there are so many di­ Cox & Knight—the Macluritina and the verse anatomies represented in limpet fami­ Trochina. In that classification the extinct GALAPAGOS RIFT LIMPET NEOMPHALUS 311 superfamilies in the suborder Macluritina cavity, and both afferent and efferent mem­ were the Macluritacea and Euomphalacea; in branes are short. ..." I have found that this the suborder Trochina there were four extinct latter condition is true of several other superfamilies: Platyceratacea, Microdomata- trochacean groups, as will be discussed fur­ cea, Anomphalacea, and Oriostomatacea. In ther in a separate paper (McLean, in prepara­ addition there were five superfamilies of tion). "doubtful subordinal position," for which sin­ All three of these different expressions of gle gills were likely: the Clisospiracea, the trochacean gill have in common the trans­ Pseudophoracea, Craspedostomatacea, verse pallial vein, an additional conduit to the Palaeotrochacea, and Amberleyacea. These afferent ctenidial vessel, requiring at least a represent major evolutionary lines for which short afferent membrane for support (except there is no direct information about their anat­ in Umbonium). The left gill of the trochacean omies. Implicit in the ranking of these groups differs in this way from the left gill of the as families and superfamilies is the assump­ pleurotomariid, which lacks the transverse tion that they had anatomical differences pallial vein and thereby has far less efficient comparable to those that distinguish the living circulation to the ctenidium. The trochacean families for which the anatomy is known. Was pallial complex has evidently been highly ef­ there in fact as great a diversity in anatomies fective from its inception, for the Trochacea as is implied by the number of available are the most successful of living archaeo- supraspecific categories? gastropods in numbers of extant species and In the Trochacea, the only superfamily of diversity of habitat. The extent of adaptive the suborder Trochina recognized as living, radiation possible for a group with the many authors (Risbec, 1939, 1955; Yonge, trochacean pallial complex has probably been 1947; Clark, 1958; Graham, 1965) have found attained. the structure of the ctenidium to be virtually The anatomical similarity of trochacean identical among species examined in all families is a remarkable fact, considering the trochacean families, including the Trochidae, diversity of shell shape, shell structure, and Stomatellidae, Turbinidae, and Phasianelli- opercular structure. The close anatomical dae.4 In its most familiar condition the relationships between families with nacreous trochacean ctenidium has a free tip with a interiors and the Skeneidae and Phasianelli- strong ventral skeleton and gill leaflets of dae, in which the primitive nacre is replaced equal size on both sides of the axis. Posterior by lamellar aragonite, would seem to belie the to the free tip about % the length of the frequently emphasized principle that shell ctenidium is supported by both dorsal afferent structure is a conservative character (for and ventral efferent membranes (Fretter & example, Batten, 1972, 1975). It is entirely Graham, 1962, figs. 53, 170). Here the leaf­ possible that many of the extinct groups could lets on the right side of the axis, where there is have had anatomies that would place them in more space, are larger than those of the left the Trochacea. The diversity of shell form in side, which are confined in a deep narrow the Trochacea is broad enough to encompass chamber (see Yonge, 1947, fig. 25). The the extremes of shell shape in some, number of leaflets in the deepest reaches of though not all, of the extinct superfamilies. this chamber may be reduced compared to The problem can be approached by asking those on the right. There are two modifica­ how the shell features in extinct groups would tions of this basic plan, that of Umbonium impose functional constraints upon their (Fretter, 1975) in which the entire gill is anatomies. monopectinate and fused to the mantle wall The Trochacea are dated from the Triassic throughout its length, and that noticed in by Knight et al. (1960: 247), but there is no Margarites (Fretter, 1955: 161) in which "the clear argument in the literature to exclude long aspidobranch gill lies freely in the mantle many older extinct families or even super-

4The Skeneidae, doubtfully considered trochaceans a short time ago (Fretter & Graham, 1962: 618), are now shown to have trochacean anatomy (Fretter & Graham, 1977: 81). I have examined the pallial complex in Liotiidae and have found a gill condition like that described by Fretter (1955:161) for Margarites. The Seguenziidae, however, despite the nacreous interior and modified rhipidoglossate radula (Bandel, 1979) have, in addition to the right subocular peduncle often occurring in trochids (see Crisp, 1981), a very large penis behind the right cephalic tentacle, as well as a fully monopectinate ctenidium (personal observation on a preserved specimen). This suggests, pending study of the internal anatomy, that mesogastro- pod-like specializations in the reproductive system have been attained and that a superfamily apart from Trochacea may be required. 312 MCLEAN families from the Trochacea. In Appendix 1, I The Euomphalacea, along with the Maclu­ show that a Permian group assigned to the ritacea, have been regarded as comprising Craspedostomatacea cannot be distin­ the archaeogastropod suborder Macluritina guished from extant trochacean Liotiidae, (Knight et al., 1960). Yochelson (manuscript) which suggests that the trochacean anatomy provides arguments that a close affinity be­ was well established in the Paleozoic. tween the two groups is no longer tenable and The trochaceans share so many characters that subordinal separation can be justified. A with the living Pleurotomariidae—nacreous suborder Euomphalina is therefore necessary interior, left kidney a large papillary sac, spiral to include the superfamily Euomphalacea caecum in the stomach, paired auricles, skel­ and the new superfamily Neomphalacea. etal rods in the ctenidial filaments, large Formal proposal of the new suborder is given paired hypobranchial glands—that their deri­ in the concluding section of this paper. The vation from a pleurotomariacean stock is read­ Macluritacea are discussed further in Appen­ ily understood (Fretter, 1964,1966). However, dix 1. the pallial condition of the Trochacea with the In the section that follows, I summarize transverse pallial vein is not what would re­ what is known of the Euomphalacea, with a main after a change amounting to little more particular effort to contrast the group with the than the loss of the right ctenidium. Trochacea. This is followed by a review of the Between the dibranchiate Pleurotomariacea recent work that proposed a filter-feeding and the unibranchiate Trochacea, Neom- mode for the Euomphalacea. phalus is the only living form that is transi­ tional in having a single bipectinate ctenidium Current Understanding of the Euomphalacea with supporting skeletal rods in the filaments, (Fig. 13) no afferent support, and thereby no additional afferent conduits to the auricle.5 Except for its Diagnosis: Shell low-spired to discoidal, modification for filter feeding, the neomphalid broadly umbilicate, some genera open-coiled; ctenidium represents what remains after the coiling dextral, some discoidal genera with the loss of the right ctenidium of a pleurotomaria­ coiling rising slightly above the apical whorl cean. With or without the filament elongation, rather than descending below; peritreme the pallial condition of Neomphalus, if it ex­ complete, upper lip trace usually sinuous but isted in a coiled shell, would be an alternative not with slit or selenizone; aperture radial, its anatomy that could provide an explanation for plane passing through the coiling axis; the anatomies of some extinct Paleozoic operculum (where known) calcified, external groups. This pallial complex, like the trocha­ pattern multispiral, inner surface with adventi­ cean pallial complex, would also impose con­ tious layers. straints upon the diversity attained by adap­ Included Families: Euomphalidae de tive radiation in some extinct groups. Koninck, 1881 (Middle Ordovician to Trias- As discussed in the section that follows, sic); Euomphalopteridae Koken, 1896 (Siluri­ paleontologists have recently hypothesized an); Oriostomatidae Wenz, 1938 (Upper Silu­ that filter feeding was the likely feeding mode rian to Lower Devonian); Omphalocirridae in the extinct Macluritacea and Euomphala­ Wenz, 1938 (Devonian); Omphalotrochidae cea. The neomphalid ctenidium provides a Knight, 1945 (Devonian to Upper Triassic); mechanism by which these archaic gastro­ Weeksiidae Sohl, 1960 (Triassic to Cretace­ pods could have been filter feeders. Apart ous). from the ease with which the neomphalid The above diagnosis reflects an altered ctenidium may be invoked to account for filter concept of the Euomphalacea, which is con­ feeding, there are clues about the coiled sistent with the paleontological literature that predecessor in the shell, for Neomphalus has has appeared since the last attempt at full a coiled phase in its first postprotoconch classification by Knight et al. (1960). They whorl. The ontogeny of Neomphalus provides recognized three constituent families (Heli- clues to its phylogeny. My theory is that the cotomidae, Euomphalidae, and Omphalo­ Neomphalidae are limpet derivatives of the trochidae) in contrast to six recognized earlier Euomphalacea. by Wenz in 1938 (Euomphalidae, Omphalo-

5A short afferent membrane is present in both neritaceans and the acmaeid patellaceans; both groups also differ from the Pleurotomariidae is lacking skeletal rods in the ctenidial leaflets (Yonge, 1947; Fretter, 1965). The cocculinid gill is not bipectinate and there are no skeletal rods (Thiele, 1903). GALAPAGOS RIFT LIMPET NEOMPHALUS 313

FIG. 13. Euomphalacean shells. A) Euomphalus pentanguiatus J. Sowerby, 1814, Carboniferous (Euom­ phalidae), x0.9. B) Straparollus iaevis (Archiac & Verneuil, 1842), Devonian, with attachment scars for shell fragments (Euomphalidae), x1.5. C) Amphiscapha reedsi (Knight, 1934), Pennsylvanian (Euomphalidae), x1.1. D) Serpuiospira centrifuga (F. A. Roomer, 1843), Devonian (Euomphalidae), xl.1. E) Oriostoma coronatum Lindstrdm, 1884, with operculum (identified by Lindstrdm to genus) in lateral view, Silurian (Oriostomatidae), x1.7. F) Beraunia docens (Pernor, 1903), Silurian (Oriostomatidae), xl.1. G) Euom- phalopterus alatus (Wahlenberg, 1821), Silurian (Euomphalopteridae), x0.6. H) Omphaiotmchus whitneyi (Meek, 1864), Permian (Omphalotrochidae), xl.1. I) Weeksia iubbocki Stephenson, 1941, Cretaceous (Weeksiidae), x1.7. After Knight et al. (1960), except operculum in E, after Lindstrdm, 1884, and G, after Linsley et al., 1978. cirridae, Platyacridae, Cirridae, Oriostomati­ cussed in Appendix 1. In the absence of an dae, Poleumitidae, and Macluritidae). Two overall revision of the Euomphalacea, the im­ recognized by Wenz—-the Omphalocirridae portant changes since 1960 may be sum­ and Oriostomatidae—are now returned to the marized as follows: list, Of the other families recognized by Wenz, Ompha/ocirrus was regarded by Wenz Platyacridae and Cirridae are here regarded (1938) as a sinistral euomphalacean, but by as trochacean (see Appendix 2), Poleumiti­ Knight et al. (1960) as macluritacean; Yochel- dae is synonymous with Euomphalidae son (1966) returned it to the Euomphalacea (Knight et al., 1960) and Macluritidae is dis­ (Euomphalidae) as a dextral form with the 314 MCLEAN

spinose projections on the under rather than estimate the number of euomphalacean taxa. the upper side; Linsley (1978a) independently Additional genera have been proposed since proposed a family Omphalocirridae to include 1960, and there are several entries per year also the genus Liomphalus (Fig. 14), which in the Zoological Record pertaining to the lacks the spinose projections, neglecting to group. In the monographic series on Permian note that Wenz (1938) had previously pro­ gastropods of the southwestern United States posed the family. (Yochelson, 1956, 1960; Batten, 1958), 45 Euomphalopterus (Fig. 13G) had been bellerophontacean species, 32 pleurotomari­ treated as pleurotomariacean, until its periph­ acean species, and 31 euomphalacean spe­ eral frill was no longer regarded as the site of cies were treated. All the other archaeogas- a selenizone by Linsley et al. (1978), who tropods (Patellacea, Trochonematacea, transferred its family to the Euomphalacea. Pseudophoracea, Anomphalacea, Craspedo- Oriostoma (Fig. 13E), with its multispiral stomatacea, and Platyceratacea) together operculum and nacreous interior, was given totaled only 21 species. It is therefore clear family and superfamily status in the Trochina that the Euomphalacea comprised a major by Knight et al. (1960); Linsley (1978a) sug­ share of the Paleozoic gastropod fauna. gested the transfer of Oriostomatidae to the Shell characters: Shell structure has here­ Euomphalacea, in which it had been previ­ tofore been an important part of the diagnosis ously placed by Wenz (1938). Opercular for the Euomphalacea, but it is omitted here characters support this assignment, as dis­ because the admission of the nacreous Orio­ cussed in the section that follows. stomatidae (Lindstrom, 1884; Knight et al., Euomphalid genera of the Mesozoic in­ 1960) changes the previous concept that the cluded by Knight et al. (1960) require further Euomphalacea were entirely non-nacreous. attention: some may need to be reassigned to As discussed above, the inclusion of families the Trochacea. Sohl (1960) proposed the with different shell structure is currently ac­ euomphalacean family Weeksiidae for three cepted in the Trochacea. Thus, the inclusion biangulate, discoidal genera—Weeksia (Fig. of nacreous and non-nacreous families in the 131), Discohelix, and Amphitomaria—differing Euomphalacea is not without precedent. from euomphalids in having a prosocline up­ B0ggild (1930: 301), in his classic survey of per whorl surface. He also noted that Hippo- the shell structure of mollusks, reported on campoides is a magilinid (i.e., coralliophilid). I the Euomphalidae as follows: "In the shells of assign Anosostoma, which had a greatly ex­ this old family the aragonite is, of course, panded final lip (Fig. 18B) to the trochacean never preserved but it seems to have existed Liotiidae in Appendix 2; no genera with ex­ originally. In most members examined by me panded apertures remain in the Euomphala­ there is a prismatic layer which is sometimes cea. rather regular and which indicates that the Yochelson (manuscript) removes Lesueu- shell, in such instances, must have pos­ rilla (Fig. 15A) and other genera with a slit or sessed an upper calcific layer." Knight et al. slit-like feature on the upper lip to the Pleuro- (1960: 189) essentially repeated B0ggild's tomariacea, and suggests that all such gen­ remarks in their superfamilial diagnosis. era should be reconsidered. Rohr & Smith The calcific layer need not have great taxo- (1978) have treated Odontomaria (Fig. 15C) nomic significance, for B0ggild (1930: 298) as pleurotomariacean. I propose that Helico- noted that it "must be said to be a rather ac­ toma (Fig. 15D) with its elevated slit be in­ cidental element," for it occurs "in a great cluded in this transfer, thereby removing the number of families," and may be lacking alto­ Helicotomidae of Knight et al. (1960) from the gether in some genera within families where it Euomphalacea. Transfer of such genera to is otherwise known. the Pleurotomariacea is in essence a return to Shell structure would be an extremely use­ the classification of Wenz, who associated ful character in archaeogastropod classifica­ them with the raphistomatid pleurotomari- tion if it were always possible to determine the aceans. original structure of fossil shells. Little can be The Euomphalidae have been reduced said of most Paleozoic and Mesozoic genera since 1960 by the removal of groups men­ and nothing can be established for those of tioned above. The content of the Omphalotro- the Cambrian and Ordovician. Presumably, chidae (Fig. 13H) remains unchanged. as in the Trochacea, nacreous interiors would It is beyond the scope of this review even to be primitive in the Euomphalacea, persisting GALAPAGOS RIFT LIMPET NEOMPHALUS 315 only in the family Oriostomatidae, a group un­ known past the Devonian,6 Although the range of possible shell forms in the Trochacea overlaps that of the Euom- phalacea (see Appendix 2), the euom- phalaceans are generally tower spired. Some, like the genus Serpulospira (Fig. 13D), are open-coiled, defined by Yochelson (1971; 236) as "shell forms that fail to have some or all of the whorls in contact but that do not obviously deviate from logarithmic factors in rate of coiling." Open coiling occurs with some frequency in the Euomphalacea, but in a review of living forms that are open-coiled. Rex & Boss (1976) reported no trochaceans with this mode of coiling, The diagnosis for Euomphalacea given here omits reference to the mode of coiling as either orthostrophic or hyperstrophic, as in Knight et al. (1960). Hyperstrophic coiling was defined by Cox in Knight et al. (1960; 131) as: "dextral anatomically, but shell falsely sinis­ tral. ..." This is a concept easily understood in conspirally coiled forms in which there is FIG. 14. Liomphalus north! (Etheridge, 1890), dextral anatomy within a sinistral shell, as di­ Devonian, Lilydaie Limestone, Lilydale, Victoria, agrammed by Cox in Knight et al. (1960:111) Australia. Showing the omphalocirrid operculum in for the ampullariid genus Lanistes,7 but it is place and coiling differences attributed to sexual here (on the advice of Yochelson) considered dimorphism by Linsley (1978a). A) Apertural view of as an inappropriate term to describe the coil­ specimen thought to be an immature female, di­ ing in such discoidai euomphalacean genera ameter 20 mm, coiling essentially orthostrophic. B) as Beraunia (Fig. 13F), Amphiscapha (Fig. Oblique apical view of specimen considered a ma­ ture male, diameter 75 mm, operculum in place, 13C) and Liomphalus (Fig. 14), in which the coiling "hyperstrophic." Photos courtesy R. M. coiling rises slightly above the apex instead of Linsley, specimens in the National Museum of below it. Living gastropods that are anatomi- Victoria. caly dextral have an operculum with a coun­ terclockwise spiral on the external surface this and similar "hyperstrophic" genera for (Pelseneer, 1893; Robertson & Merrill, 1963). which opercula are unknown were anatomi­ Opercula with a counterclockwise spiral are cally dextral. known in such euomphalacean genera as "Hyperstrophic" coiling has been used as a Liomphalus (Fig. 14), providing the evidence generic-level character in some members of generally accepted by paleontologists that the families Euomphalidae, Omphalocirridae

"Guinn (1981) has suggested that the nacrccur. Se-cucnziicae dive- ;-.u.c B.mdol, 1979) could have been derived from the Omphafotrochidae. a family here included in the Euomphalacea. Because nacre is unknown in the Omphalotrochidae, such a derivation would require the unlikely reversion to nacre. 'Hyperstrophy is known in two living mesogastropod familioi.—in the larval stages of architectonicids and in the African ampullariid genus Lanistes (see Wenz, 1938). In architectonicids it is normally limited to the planktotrophic veliger stage (Robertson, 1964), although rare abnormal specimens have been found in which hyperstrophy persists in the adult (Robert­ son & Merrill, 1963). Normally the coiling changes to orthostrophic in the first teleconch whorl. In Lanistes it is apparent that these moderately high-spired forms carry the shell directed to the left rax as in sinistral gastropods, but that water currents move in the mantle cavity from left to right as in dextral gastropods (Lang, 1891: 368, fig. 21, copied in part by Cox in Knight et al., 1980, fig. 87). Andrews (1965: 71) studied Lanistes and noted that its mantle cavity is deeper than that of orthostrophic members of the family, but she did not discuss the functional advantage of hyperstrophy in Lanistes. Hyperstrophy raises some questions, for, according to descriptions of torsion (Crofts, 1955), the normal course of development leads to dextral orthostrophic coiling. Crofts showed that in the archaeogastropods Haiioiis, Patella, and Calliosioma, the first phase of torsion involves a delayed development of the left compared to the right post-torsional retractor muscle, which imposes an immediate asymmetry upon the protoconch, causing the direction of coiling to proceed in the usual dextral manner. In sinistral gastropods the anatomical sinistrality may be traced to the first stages of cleavage, as recently reviewed by Verdonk (1973). Discussions of torsion (Lever, 1973, and references therein) make no mention of hyperstrophy. How hyperstrophy in architectonicids and Lanistes can follow torsion is worthy of further investigation. 316 MCLEAN

and Oriostomatidae. Linsley (1978a) consid­ Radial apertures are characteristic of all ered that the four omphalocirrid species he families in the Euomphalacea. Apertures in studied showed sexual dimorphism—a rea­ the Trochacea tend to be oblique, or—in sonable conclusion based on the equal num­ Linsley's terminology—tangential, defined as bers of supposed male and female morpho- "an aperture whose plane is tangent to the types in each species. Those he interpreted body whorl," so that it and the ventralmost as females (Fig. 14A) tended to have iso- part of the body whorl lie in one plane. strophic to orthostrophic coiling, in contrast to Multispiral calcareous opercula are known the decidedly "hypertrophic" males (Fig. in the families Omphalocirridae (Fig. 14) and 14B). This intraspecific variability in coiling Oriostomatidae (Figs. 13E, F). Other euom­ direction indicates that there was no anatomi­ phalacean families may have had multispiral cal difference between orthostrophic and opercula that were uncalcified, or their original "hypertrophic" euomphalaceans. aragonitic opercula may have preserved There are no families or genera in the poorly compared to the calcitic shell. Such Euomphalacea in which there is a thickened mineralogic differences between shell and final lip or abrupt change in coiling direction, operculum are known in some Recent tur- as in the Trochacea (see Appendix 2). binids and neritids (Adegoke, 1973). The The diagnosis for the Euomphalacea in omphalocirrid operculum is best known in Knight et al. (1960, p. 189) included the provi­ Liomphalus northi (Fig. 14). It has recently sion: "commonly with channel presumed to been described by Yochelson & Linsley be exhalant occupying angulation on outer (1972) and Tassell (1976: 9). This type of part of upper whorl surface." Yochelson operculum varies in thickness, is disc-shaped, (manuscript) now notes that most euom­ slightly concave externally, beveled to fit tight­ phalaceans do not have a prominent shoulder ly within a circular aperture, and has numer­ and that in those that have an angulation the ous externally visible volutions and internal shell is thickened in that area and there is no laminar layers. It is quite similar to the Cyclo- interior channel to be regarded as an exhalant spongia operculum, an operculum first route. Thus, this provision of the diagnosis is thought to be a sponge, but redetermined by no longer included. It is to be noted that the Solem & Nitecki (1968) as a gastropod oper­ growth line on the upper lip of many euom­ culum from an unknown shell.8 External sur­ phalaceans is often sinuous and opisthocline, faces of opercula are known in two other as in Omphalotrochus (Fig. 13H), although omphalocirrids treated by Linsley (1978a). Weeksia (Fig. 131), with a prosocline lip, is an The oriostomatid operculum is known in exception. The trochacean lip is usually Beraunia (Fig. 13F) (see also Knight, 1941, prosocline. pi. 80) and in Oriostoma (Fig. 13E) (see also Euomphalacean protoconchs were de­ Lindstrom, 1884, pi. 17, and Kindle, 1904, pis. scribed by Yochelson (1956: 195) as "com­ 11, 14). Externally, the oriostomatid oper­ monly discoidal," but to my knowledge have culum is conical, in some cases higher than not been illustrated. Dzik (1978) illustrated broad, the central nucleus projecting, the suc­ protoconchs of some Ordovician gastropods ceeding whorls descending and having raised that resemble those of modern archaeo- edges. The mode of formation of both the gastropods. However, it is not certain whether omphalocirrid and oriostomatid opercula any of those he figured are referable to the would be similar, with accretions at the edge Euomphalacea. produced in the opercular groove on the ani­ The concept of the "radial aperture" was mal's foot, and adventitious layers added on introduced by Linsley (1977: 196), defined as the underside, as it rotates in a clockwise di­ "an aperture whose plane passes through the rection to produce the counterclockwise coil axis of coiling and thus lies along a radius of the external surface. These opercula are from the coiling axis to the shell periphery." unlike the turbinid operculum, in which a

^Yochelson & Linsley (1972) considered that the Cyclospongia operculum matches the operculum described by Tyler (1965: 348, pi. 48, figs. 19-25) and assigned by Tyler to his species Turbinilopsis anacarina. That assignment violates the well-reasoned hypothesis of Solem & Nitecki that the shell of Cyclospongia must have been a 'planorbiform, depressed helicoidal, or helicoidal shell possessing a circular aperture, deep sutures. ..." Turbinilopsis as applied by Tyler is assigned to the Anomphalacea. In my opinion, such a shell is wholly inappropriate for the Cyclospongia operculum because it has a tangential aperture and lacks an umbilicus. I cannot agree with Yochelson & Linsley (1972) that an operculum as discrete as those of Liomphalus and Cyclospongia can be convergent in widely different families. I am certain that a euomphalacean shell eventually will be found for the Cyclospongia operculum. GALAPAGOS RIFT LIMPET NEOMPHALUS 317

paucispiral or multispiral pattern is preserved that euomphalaceans were sessile animals on the inner surface but is obliterated on the resting on the base of the shell. external surface where it is enveloped by the Peel (1975a) also discussed the probability animal's foot. The omphalocirrid and orio- that open-coiled Paleozoic gastropods were stomatid opercula differ from the trochid, sedentary. He contrasted open-coiling with turbinid and liotiid opercula in depositing ad­ the uncoiling of higher-spired forms, which ventitious layers on the internal surface. Thus, also suggests a sedentary existence (see the euomphalacean and trochacean oper­ also Gould, 1969). He concluded that "Paleo­ cula, though both multispiral, are entirely dif­ zoic gastropods were more diverse in their ferent. There is convergence in shell form in feeding habits than comparison with extant the Trochacea and Euomphalacea, but the gastropods would suggest." distinction may be clearly drawn between Linsley (1977, 1978b, 1978c, 1979) devel­ those members in which opercula are known. oped the concept of the radial aperture—in Feeding and locomotion: During the pre­ which the plane of the aperture would pass ceding decade a number of papers have con­ through the coiling axis. Gastropods with sidered possible modes of locomotion and radial apertures would have difficulty balanc­ feeding in the Euomphalacea. The theme has ing the shell over the cephalopedal mass. His been developed that these gastropods rested "law of radial apertures" states (1977: 109): with the aperture perpendicular to the sub­ "Gastropods of more than one volution with stratum, unlike the trochaceans in which radial apertures do not live with the plane of the shell is balanced over the cephalopedal the aperture parallel to the substrate. Most mass and the aperture maintained in a posi­ typically it is perpendicular to the substrate." tion parallel to the substratum. Few living gastropods have radial apertures. Yochelson (1971) discussed open coiling In one major example, the Architectonicidae, and septation in the Devonian euomphalid the animals are mostly sedentary and "usual­ Nevadispira (which is similar to Serpulospira, ly lie with the shell on the substrate" (Linsley, Fig. 13D). He suggested that it had a seden­ 1977). For the Euomphalacea he stated tary life mode because an animal with open (1977: 204): "I suggest that all had adopted a coiling would have great difficulty balanc­ rather atypical gastropod posture of lying with ing the shell for locomotion, the septation the shell flat on the sediment, rarely if ever that shortened the body mass would further hoisting it above the cephalopedal mass in hamper locomotion, the open coiling would the stance associated with the majority of increase the area of contact with the substrat­ modern forms." The only possible means of um, and the "hyperstrophic" coiling would locomotion would be what Linsley has called raise the aperture above the sediment. Thus, "shell dragging." In view of the sedentary this "would appear to be a natural response in habit, Linsley has considered suspension shape change for a coiled animal living a feeding to be the most likely feeding mode, sedentary life on a mud bottom." He sug­ "either by filtering with their gill(s) or by cast­ gested that euomphalids may have been de­ ing mucous nets" (1979: 251). posit feeders rather than herbivores and that Schindel (1979) found encrusting epibionts the open-coiled members "may have further on the exposed apical cavity surface of the specialized toward ciliary feeding." This sug­ "hyperstrophic" euomphalid Amphiscapha gestion was in contrast to the traditional (Fig. 13C), whereas the basal surfaces were dictum that all archaeogastropods are herbi­ free of encrustations. This indicates that the vorous. basal surface was never exposed as would Linsley & Yochelson (1973) discussed happen if the life mode involved shell balanc­ Devonian members of Straparollus (Fig. 13B) ing. This provides further confirmation for and Euomphalus that had the habit of attach­ Linsley's principle. ing foreign matter to the shell in a way com­ I can here add the observation that the parable to that of the modern Xenophoridae. oriostomatid operculum precludes locomotion They concluded (1973: 16) that these euom­ by shell balancing in that group. Shell-balanc­ phalids were unlikely to have balanced the ing gastropods use the operculum as a pro­ shell like trochaceans, it being "most unlikely tective pad placed between the shell and the that Straparollus laevis could have held its foot. In the turbinids the dorsal surface of the shell motionless in the normal carrying posi­ foot envelops the external surface of the tion for the several hours required" for implan­ operculum, keeping it smooth, or in some tation of objects. This was further evidence species producing intricate sculpture. The 318 McLEAN turbinid operculum is not so thick that it can­ probably was the locus of an anus as in the not be carried in the usual position between Macluritacea, and the distance of this keel the foot and the shell. However, the conical from the suture would have allowed ample oriostomatid operculum, which may be higher space in the mantle cavity for paired ctenidia." than broad (Fig. 13E), was not enveloped by Cox & Knight (1960: 262) took a position on the foot (which would have altered its sharp middle ground: "Right ctenidium inferred to sculpture) and is too large and sharply point­ have been reduced and in some forms pos­ ed in the center to have been carried between sibly absent." Golikov & Starobogatov (1975) the foot and the shell during locomotion. included the "Order Macluritida" among the Extinctions: Euomphalacean genera and dibranchiate gastropods. species proliferated in the Paleozoic. Few Linsley (1978c: 440) suggested that stocks survived the mass extinctions at the Macluritacea and Euomphalacea "had only close of the Permian. Vermeij (1975, 1977) one inhalant and one exhalant stream and correlated their further decline in the Meso­ probably only a single gill," and that the shape zoic with the appearance of such shell-crush­ of the aperture "makes sense if these forms ing predators as teleosts, stomatopods and did not undergo torsion." Thus, they "there­ decapod crustaceans. The broadly umbilicate fore should not be considered gastropods." or openly coiled euomphalacean shells are Linsley's theory has not as yet been fully de­ poorly constructed to resist crushing. There tailed. It seems to me, however, that the are few broadly umbilicate forms among euomphalacean operculum strongly suggests modern marine gastropods. Shells tend to be gastropod affinities. sturdier, with narrower apertures, often hav­ Yochelson (manuscript) now advocates the ing such modification as apertural dentition or removal of genera with a slit from the Euom­ spiny external surfaces to strengthen the phalacea and finds no indication of an ex­ shell. halant canal in those that remain; he therefore More recently Thayer (1979) has discussed finds no evidence of paired gills. a trend in the evolution of marine benthic My theory for the anatomical reconstruction communities. Paleozoic communities on soft of the Euomphalacea includes torsion, allows sediments were dominated by immobile sus­ both orthostrophy and "hyperstrophy," and pension feeders such as articulate brachio- reconstructs them as unibranchiate, as pods, dendroid graptolites, tabulate and originally proposed by Knight (1952). Peel rugose corals, bryozoa, cystoids, and blas- (1975a: 218) understood that bipectinate toids. In the Mesozoic and Cenozoic, the ctenidia modified for filter feeding would entail soft-bottom benthic communities are domi­ some essential differences from the ctenidia nated by infaunal deposit feeders that include of modern filter feeders: "The effects of this protobranch bivalves, irregular echinoids, difference in the structure or even number of certain crustaceans, holothurians, and an­ ctenidia upon the form of a mantle cavity nelids. The disruption or bioturbation of the adapted to ciliary feeding are perhaps impos­ sediments by the large infaunal deposit feed­ sible to estimate. It is certainly possible that ers would foul or bury the soft-substrate sus­ another arrangement of ctenidia and mantle pension feeders, particularly their juvenile cavity was required and that this was at vari­ stages. This, in addition to their vulnerability ance with the elongate ctenidium and long to shell-crushing predators, could also ac­ narrow mantle cavity of the Recent species." count for the demise of the soft-substrate liv­ The neomphalid mantle cavity now provides ing Euomphalacea, a group not mentioned by the best model for the reconstruction of the Thayer. euomphalacean mantle cavity. There is little Previous interpretations of euomphalacean essential difference between the filter-feed­ anatomy: The Euomphalacea have been ing mantle cavities of calyptraeid limpets and variously interpreted as either dibranchiate or the coiled turritellids. The placement of the unibranchiate. Knight (1952:40), in his classic neomphalid feeding mechanism within the paper on primitive gastropods concluded that eumphalacean shell is equally plausible. I in "hypertrophic" forms there was "very little therefore accept the filter-feeding mode of life room for a right ctenidium" and assumed that for the euomphalaceans recently suggested it and the associated organs had been lost. by Yochelson, Peel, and Linsley. Yochelson (1956: 195) considered that the Apart from the ease with which the Euomphalacea were dibranchiate: "The char­ neomphalid mantle cavity could be construed acteristic keel on the upper whorl surface as having been possible within a coiled shell, GALAPAGOS RIFT LIMPET NEOMPHALUS 319 there is a strong correlation between the phalacea, my supposition is that regular coil­ musculature and ontogenetic development of ing continues, but the alignment of the body the shell in Neomphalus and that of the relative to the coiling axis shifts by 90°. Me­ euomphalaceans, as discussed in the section chanical considerations require that the major that follows. area for muscular insertion on any discoidal shell be on the inner, columellar wall. Muscle Neomphalus as a Euomphalacean Derivative attachment on any other surface would be un­ necessary. For an animal oriented to the sub­ Evidence has been presented in the pre­ stratum in a flat-lying shell, this will mean that ceding section that their radial apertures pre­ the right side of the body assumes the entire cluded the euomphalaceans from balancing muscle attachment function. There is no need the shell over the cephalopedal mass. Thus for a left columellar muscle. The left side of they had to rest the shell on its base, which the body is therefore available for a long, was concave for orthostrophic shells or flat for deep mantle cavity. "hypertrophic" shells. This is in complete Neomphalus is the logical result of the con­ contrast to the life mode of the trochaceans. version of the euomphalacean body plan to Trochaceans have tangential apertures— the limpet form. One of the most significant the tangential aperture exposes less body features of Neomphalus is the occlusion by surface than the radial aperture when the ani­ columellar muscle of the entire right side of mal is attached to a hard substratum. The the body posterior to the neck. The columellar shell is balanced over the cephalopedal mass muscle is lateral to the body mass, just as it and the columellar muscle is ventral to it dur­ must have been in a euomphalacean. ing locomotion. Even when retracted within Veliger stages of all gastropod larvae are the shell, the cephalopedal mass remains similar in having the shell balanced over the dorsal to the columellar muscle, which means cephalopedal mass. Post-veliger euom­ that the animal actually rests upon its left side phalaceans would be motile, would balance when the shell is resting upon the base. Thus the shell, and would feed by grazing. Growth the head always maintains a position that is of the columellar muscle would be pro­ perpendicular to the axis of coiling. When the grammed to shift the muscle to the right of the animal extends, a twist in the alignment of the cephalopedal mass, causing the animal to head of approximately 45° is necessary to lose the shell-balancing capacity and assume balance the shell, tilting the spire up and to the filter-feeding mode. the right rear. In its protoconch and first postprotoconch What can be said about the position of the whorl, the neomphalid animal must carry its head relative to the axis of coiling in the ex­ shell with the coiling axis and plane of the tinct euomphalaceans? In the absence of shell aperture parallel to the substratum. Its trans­ balancing, there is no reason to assume that formation to the limpet form involves cessa­ the cephalopedal mass of mature animals tion of coiling and a 90° shift of the shell to was aligned to the coiling axis. In normal feed­ place the coiling axis perpendicular to the ing posture the head of any animal needs to substratum. The same 90° shift in the place­ be balanced relative to the substratum. If the ment of the coiling axis is presumed to occur head and body of a euomphalacean animal in in the ontogeny of all the extinct euomphala­ retracted condition was aligned toward the ceans in which the regular coiling continues. coiling axis, a 90° twist would be required to The euomphalacean alters the orientation of place it in a feeding posture, an unnecessary the animal within the shell; the neomphala- requirement for an animal that never needs to cean effects the change by growth stoppage balance its shell. Moreover, the feeding pos­ along the columellar lip; in both cases the ini­ ture of a filter-feeding gastropod is one in tial coiling axis becomes perpendicular to the which the head remains within the shell aper­ substratum. This is the essential requirement ture, as in Turritella. Most likely the head in euomphalacean and neomphalacean on­ would be permanently aligned relative to the togeny that distinguishes these superfamilies substratum. The columellar muscle would from all other living archaeogastropods, therefore be lateral rather than ventral to the whether coiled or limpet derivatives of coiled cephalopedal mass. Modern gastropods forms. with irregular coiling have abandoned coiling The relatively large size of the neomphalid and thereby dissociated the columellar mus­ larval operculum and its vestigial retention in cle from the axis of coiling. For the Euom- juvenile sizes far larger than that of other 320 McLEAN limpets is additional evidence that a coiled If Neomphalus was derived from weeksiid ancestry is phylogenetically close. The pres­ euomphalaceans, the minimal age for the ence of epipodial tentacles only near the site family would be Cretaceous. Because the of the operculum is consistent with the idea euomphalaceans were the dominant uni- that euomphalaceans were filter feeders in branchiate gastropods in the Permian, it can which the head and foot were kept within the be argued, however, that the Paleozoic, when shell in feeding position. There would be no numerous stocks were present, is the most use of epipodial structures away from the likely time of origin of the Neomphalidae. operculum in euomphalaceans. The origin of Neomphalus may have been a Entry of Neomphalus into the Rift-Vent rapid event brought about by a relatively sim­ Community ple alteration of the developmental process, one that inhibited growth along the basal por­ The rift-vent habitat has probably been tion of the columellar lip, forcing continued available over long periods of geologic time, growth to produce lip expansion and the for­ because it is likely that hydrothermal vents mation of a limpet in much the same process have accompanied tectonic movements as revealed in the ontogeny of Neomphalus. If throughout the entire history of the earth. The such an event in an euomphalacean stock oceanic rift system is global in magnitude took place near an active rift-vent site, the (Corliss et al., 1979: 108), although the full new limpet would be especially adapted to extent of hydrothermal activity along it is un­ utilize the abundant sulphur bacteria in this known. Vents have not yet been found along rocky environment. Neomphalus represents a the mid-Atlantic Rift, but at least two widely highly successful response to an abundant separated sites in the Pacific are now known. food supply, entailing no loss of body size, As stated by Spiess et al. (1980: 1424): using less calcium than that required by a "The similarity of the East Pacific Rise and coiled shell, and affording some protection Galapagos Rift fauna suggests that these from shell-crushing predators. The limpet vent communities are widespread and that conversion represented by the Neomphalidae their species are equipped with sophisticated was perhaps the only as yet untested dispersal mechanisms well suited for the de­ morphological theme in a stock already tection of the discontinuous and ephemeral specialized for filter feeding. vent conditions." This similarity also suggests The Mesozoic euomphalacean family stability of the community. Invasions of spe­ Weeksiidae, proposed by Sohl (1960), has cies from other habitats must be of rather in­ some features in common with Neomphalus. frequent occurrence. Possible barriers to new Characters shared by Neomphalus and the colonizations of the community include the Cretaceous Weeksia (Fig. 131) mentioned by differing chemical conditions, cold water Sohl (1960: 50) are: "ornament usually poorly masses separating the warm environment of developed . .. growth lines prosocline on up­ the habitat from other warm environments, per surface .. . moderately large shell with and the scarcity of hard substrates to serve as raised naticoid protoconch." The discoidal stepping stones from shallow water into a shell of Weeksia has an orthostrophic proto­ deep-sea hard-substrate environment. Mol- conch whereas the later whorls are faintly luscan predators such as sea stars and drill "hypertrophic." The early shell ontogeny of snails are not known to be present. In the Neomphalus does not include a stage having absence of these predators, the rift-vent com­ the biangulate lateral profile of weeksiid munity seems well suited to provide refuge for genera. However, I have examined speci­ an archaic molluscan group specialized for mens of the similarly constructed biangulate filter feeding. euomphalacen Amphiscapha and note that Modern filter-feeding gastropods, the tur- the earliest whorls are unsculptured. Thus the ritellids and the calyptraeids, occur in shallow postprotoconch whorls of Weeksia and water from the intertidal zone to the con­ Neomphalus can be considered far less dif­ tinental shelf, with none known from conti­ ferent than the mature teleoconch whorls. If nental slope or abyssal depths. This evidently the juvenile shells are to provide the only reflects a scarcity of sufficient suspended food characters in common, it is unlikely that the for these relatively large forms under normal direct ancestor of Neomphalus will ever be conditions at abyssal depths. A filter-feeding known. gastropod the size of Neomphalus would GALAPAGOS RIFT LIMPET NEOMPHALUS 321

have to have a shallow-water origin, from "living fossil," a term limited by Eldredge which it would make the transition to the rift- (1975) and Stanley (1979: 258) to "taxa that vent community with no interruption in abun­ have persisted for long intervals of time with dance of the food source, through rift-vent little evolutionary change and that are primi­ sites in progressively deeper water. A shal­ tive or archaic in comparison with living taxa low-water origin for the Neomphalidae is also of the same class or phylum." It can be consistent with findings by Clarke (1962) that argued that the neomphalid gill can only be no molluscan families have originated in the archaic, since it is not represented in any deep sea. Shallow water occurrences at one other family in normal marine habitats. time are known for all deep-sea mollusks with If there were a fossil record of the family, continuous Paleozoic to Recent fossil rec­ the Neomphalidae could be compared to the ords. nautiloid cephalopods, the neopilinid mono- There is precedence for the interpretation placophorans, the pleurotomariid archaeo- of a rift-vent community member as a relict gastropods, and the abyssochrysid loxone- species. Newman (1979) considered the mataceans, recently added to the list of living stalked barnacle Neolepas zevinae, which he fossils by Houbrick (1979). These families named from hydrothermal vents on the East were once diverse in shallow seas of the Pacific Rise at 21° N latitude (see Grassle et Paleozoic and Mesozoic but survive now at al., 1979; Spiess et al., 1980), to represent a the lower limits of the continental shelf to stage of barnacle evolution attained in the the abyss. Each family is still represented by Mesozoic. several species. Speciation events have ap­ Newman's hypothesis for the origin of parently kept pace with extinctions. The aver­ Neolepas is as follows (Newman, 1979:153): age duration—the Lyellian curve—for marine "Habitat also favors the interpretation that gastropod longevity is about 10 million years Neolepas is a relict form, having found refuge (Stanley, 1979: 237). Even if a neomphalid near deep, hydrothermal springs. Such a species could endure as long as 20 or 30 mil­ refuge may have been attained in the late lion years, numerous speciation events Mesozoic when predation pressures on ses­ should have occurred, and other species (or sile organisms are inferred to have dramatic­ genera) are likely to be living now at other ally increased. Though immigration into the rift-vent systems. An effective dispersal hydrothermal environment by deep-sea mechanism for Neomphalus is unknown. This stocks is a distinct possibility, in the present is a factor that should increase its speciation case, the route appears more likely to have potential, because new colonies would stay been from relatively shallow waters of warm isolated the longer. The possibility that a and tropical seas where tectonically active single species has represented the family rifts intersect continental crust, and perhaps throughout its entire existence seems the where islands are forming along ridge crests." least plausible alternative. This explanation provides for both the antiquity and the route into the rift-vent com­ Reconstruction of Euomphalacean Anatomy munity for Neolepas zevinae. It is also the best hypothesis to account for the presence of An attempt to reconstruct the anatomy of Neomphalus in the rift-vent community. If the euomphalaceans can be based upon two origin of Neomphalus was quickly followed by models: Neomphalus and Turritella. Because submergence, as postulated by Newman for Turritella is a mostly sedentary filter-feeding Neolepas, a fossil record of Neomphalus in animal on soft bottoms (Graham, 1938; shallow water would be elusory. Fossil rec­ Yonge, 1946), there should be many paral­ ords of deep-sea mollusks are all but un­ lels. Differences between the mesogastropod known because of the solubility of calcium Calyptraeidae and the Turritellidae should be carbonate shells at abyssal depths (Berger, about equivalent to the differences between 1978; Killingley et al.,1980). Neomphalus and the euomphalaceans. According to my supposition, the origin of Coiling differences are reflected in the the Neomphalidae took place at some point orientation of the turritellid and euomphala­ between Late Paleozoic to Late Mesozoic, cean mantle cavities. The mantle cavity of the giving it an age in the range of 70 to 250 extremely high-spired Turritella has to turn million years. If a fossil record for the family like a corkscrew through at least one full could verify such an age, it could be called a whorl; that of the euomphalacean maintains a 322 MCLEAN horizontal position but has to curve to the tached about 1/3 of a whorl behind the aper­ right. It may be a requirement that filament ture and the mantle cavity would extend at tips of a bipectinate ctenidium have to relate least another third of a whorl deeper. The to a horizontally aligned food groove; the sin­ neck and head would extend forward of the gle rack of filaments of a pectinibranch filter- area of muscle attachment and would be feeder should have no difficulty relating to the broad and flattened as in Neomphalus be­ food groove, whatever the orientation. cause of compression from above and below. Although the columellar muscle of Turritella The space above is taken by the free tip to the is ventral to the cephalopedal mass as in ctenidium and the space below is taken by the motile gastropods, the extremely high-spired foot. A deeply channeled left neck groove like shell is too heavy to be balanced for locomo­ that of Neomphalus would help to keep some tion. In Turritella the early whorls are made open space at the left and to provide a rejec­ heavy and are partially filled by septation and tion and cleansing channel for the mantle deposition of callus (Andrews, 1974). A simi­ cavity. lar process of septation and deposition in the In Turritella pallial tentacles provide a early whorls is also characteristic of euom- coarse filter for the incurrent stream. In phalacean shells (Yochelson, 1971). Stability euomphalaceans, tentacles of either pallial or on soft bottoms is thus enhanced in both epipodial origin would be used for that pur­ groups. pose. Other features of the mantle cavity There are remarkable parallels between should be like those of Neomphalus: a bipec­ Turritella and the euomphalaceans in aper­ tinate ctenidium would extend the length of ture shape and structure of the operculum. In the mantle cavity, attached ventrally to the both groups the aperture is radial and the mantle skirt, the free tip emerging near the operculum multispiral. The sinuous whorl side region of columellar attachment and extend­ of Turritella marks the position of a dorsal ex- ing over the neck: the split osphradium lo­ current siphon; a similar opisthocline sinus in cated at the separation of the free tip; the the upper lip of some euomphalaceans, par­ dorsal afferent membrane lacking, so that the ticularly the omphalotrochids, can also be in­ filament tips from both sides of the gill axis terpreted as the excurrent sinus. can reach the food groove; the food groove In feeding posture Turritella lies partially extending the full length of the mantle cavity, buried on soft bottoms so that the operculum running anteriorly over the dorsal surface of nearly blocks the aperture. The exceptionally the long neck and cutting directly to the small foot (Yonge, 1946) remains contracted, mouth. sole up, directly behind the operculum (Fretter Because both Turritella and the calyptrae- & Graham, 1962, figs. 57, 64), except when ids have eyes and anteriorly directed cephalic used to clear an incurrent depression in the tentacles, it is likely that the euomphalacean substratum (Yonge, 1946, fig. 1). Continuous head would have such features, having a need inhalant and exhalant currents are maintained for greater sensory contact outside of the unless the foot and operculum are fully re­ shell than that of Neomphalus. However, the tracted. dorsal food groove precludes the presence of Placement of the neomphalid anatomy in a snout, so the most reasonable assumption the euomphalacean shell would require the is that the head and neck were structured foot to curl forward so that it comes to lie, sole much like that of Neomphalus. up, underneath the long neck, which would In Neomphalus a fecal groove extends well position the operculum so that it loosely beyond the mid-dorsal anus, the ctenidial fila­ blocks the aperture, as in turritellids. In most ments keeping the fecal groove in the mantle euomphalaceans the foot must have been skirt well separated from the food groove on contained entirely within the aperture, for the neck. The same arrangment must have there is no ventral gape in the shell. Like the obtained in the euomphalacean, the general turritellid foot, the euomphalacean foot would pattern of water currents in the mantle cavity be relatively small. Because the aperture is so being ventral to dorsal, rather than left to right. far to the side of the shell's center of gravity, The euomphalacean mantle cavity is com­ the euomphalaceans were probably no better pletely asymmetrical, extending laterally and adapted for burrowing than for locomotion. ventrally rather than dorsally over the cephalo­ The euomphalacean would have its entire pedal mass. This asymmetry would also work visceral mass deep within the coils of the to dislodge the primitive juxtaposition of the shell. The columellar muscle would be at­ rectum and ventricle, so that the complete GALAPAGOS RIFT LIMPET NEOMPHALUS 323

monotocardian condition is a necessary the muscle would enable retention of shell- consequence of the euomphalacean body balancing mobility and could account for plan. In the absence of a similar leftward dis­ some of the more high-spired euomphal­ placement of the mantle cavity, the Trochacea aceans with shell shapes that converge upon and Neritacea have remained diotocardian, those of the Trochacea (some oriostomatids, despite their loss of the right ctenidium. some euomphalids, some omphalotrochids). Although the monotocardian condition is a If the radula retained its early prominence, the likely consequence of the leftward shift of the initial grazing capacity would be retained. mantle cavity, the mesogastropod level of The relatively high-spired euomphalaceans reproductive advancement need not be. It is could have behaved like the freshwater problematic whether these features were pri­ mesogastropod Viviparus. Though quite mitive to euomphalaceans or represent an capable of normal shell-balancing, locomotion adaptation of Neomphalus to the rift-vent en­ and rasping with the radula, Viviparus also vironment. It is clear that the genital opening employs a filter-feeding stance in which the in euomphalaceans would have to be within shell lies half buried, aperture up, the the mantle cavity on the left side. If a copula- operculum partially blocking the aperture tory appendage was present, it would have (Cook, 1949; Fretter & Graham, 1978). been on the left side because this is the side The fossil chronology indicates that the close to the genital opening and there would earliest euomphalaceans were low-spired be more space for it on the left than the right. and discoidal. This suggests that the mono­ The likely immobility of euomphalaceans tocardian condition with a fully bipectinate makes it improbable that they could have ctenidium was primitive to all euomphal­ moved to copulate effectively. There is no aceans. Given this premise, many different reason to suggest that broadcast spawning expressions of the basic body plan were pos­ through an unmodified left kidney would not sible. be suitable for an immobile animal in concen­ trated shallow-water populations. Origin of the Euomphalacea If my basic assumption—that the columellar muscle is positioned to the right rather than Although Knight (1952) did not mention the ventral to the body mass of the euomphal­ Euomphalacea in his classic paper on primi­ acean—is valid, then the variable expression tive gastropods, he discussed a derivation of of "hyperstrophy" or orthostrophy can be Macluritacea from the Bellerophontacea. Two considered a result of the shift in position of years later, Knight, Batten, and Yochelson the body relative to the columellar muscle. (1954) diagrammed a phytogeny of Gastro­ The direction of coiling then becomes entirely poda in which the Macluritacea were derived a matter of convenience to elevate or lower from the Bellerophontacea and the Euom­ the aperture above the substratum as an phalacea in turn derived from the Malcurit- adaptation to particular bottom conditions. acea, a view also followed by Knight et al. Thus the hyperstrophy hypothesized for the (1960). Euomphalacea is unlike that of larval archi- Yochelson (manuscript) has a new theory tectonicids or Lanistes in the Ampullariidae, in that seems more compatible with my recon­ which the columellar muscle is always ventral struction for the Euomphalacea. He specu­ to the cephalopedal mass. This justifies the lates that they could have been derived in the rejection of the term hyperstrophy with refer­ Ordovician from a Lecanosp/ra-like pleuroto- ence to the Euomphalacea. mariacean following the loss of the right My theory predicts that ontogeny in a ctenidium in a way comparable to the sepa­ euomphalacean involves these changes: 1) rate derivation of the Trochacea. Lecanospira the columellar muscle shifts, relative to the (Fig. 15B) had previously been regarded by cephalopedal mass, from the ventral position Knight et al. (1960) as a macluritid, but in the postveliger to the right lateral position in Yochelson presents convincing arguments the adult, 2) the feeding mode changes from that it and genera like Lesueurilla (Fig. 15A) grazing to filter-feeding, which involves with a deep V-shaped notch in the upper lengthening of the gill filaments, and a corre­ aperture are best interpreted as pleuroto- sponding decrease in the relative size of the mariaceans. This group of genera was limited radula. The extent to which these changes to the early Paleozoic, none being represent­ were effected could have varied in different ed in the extensive euomphalacean fauna of lineages. An incomplete shift in the position of the Permian (see Yochelson, 1956). 324 McLEAN

FIG. 15. Early Paleozoic genera now excluded from the Euomphalacea for having a prominent raised slit or selenizone. This group of genera is now regarded {Yochelson manuscript) as the fow-spired pleuroto- martacean group ancestral to the Euomphalacea. A) Lesueurilla infundibulum (Koken, 1898), Ordovician, x1.1. B) Lecanospira compacta (Salter, 1859), Ordovician, ;<1.1. C) Odontomaria elephantina C. F, Roomer, 1876, Devonian, x0.8. D) Helicotoma planulata Salter, 1859, Ordovician, x1.6. All after Knight et al. (1960).

Like euomphalaceans, such genera are suspension to be collected into a place where low-spired and discoidal. Open coiling is the gastropod may use it. It is only when the represented in Odontomaria (Fig. 15G) (see water current is the transverse stream of the also Rohr & Smith, 1978). Lecanospira and mesogastropod that this happens" (Fretter & Lesueurilla are "hypertrophic," like some Graham, 1962: 98). euomphalaceans. This shell form, whether The possibility that the food groove in a represented in a unibranchiate or a dibranch- dibranchiate filter-feeder could take a dorsal iate gastropod, presents the same constraints route over the head to the mouth has not for locomotion already discussed. Thus these heretofore been considered. Lengthened genera were probably sedentary forms rest­ ctenidial filaments arising from both gills could ing for the most part on their flat bases. As­ converge upon a central food groove. The suming that they were dibranchiate pleuroto- food groove of Neomphalus is deflected to­ mariaceans, the question arises: could these ward the right before arching toward the forms have been filter feeders? mouth, but this could be a vestige of its primi­ The food groove of Neomphalus provides a tive mid-dorsal position. Many of the unusual relevant clue, for Neomphalus is the only features of the body plan of Neomphalus can known prosobranch in which the food groove be understood in terms of additional torsion takes a dorsal route to the mouth. In pectini- and rotation on the anteroposterior axis, as branch filter feeders and even in the trochid discussed by Fretter, Graham & McLean Umbonium the right lateral food groove has (1981), but no such shifts could account for a developed independently in several families migration of the food groove (or a correspond­ by "conversion of the tract on the right of ing ciliated tract) across the right cephalic the mantle cavity, along which the food par­ complex to a dorsal position. One way to ac­ ticles are led to the mouth, into a deep count for the dorsal position of the food gutter... which runs across the whole of the groove is to consider it a primitive character floor of the mantle cavity to a point just under shared by the dibranchiate ancestor. Thus the right cephalic tentacle" (Fretter & Graham, there is good reason to suggest that filter 1982: 100). They noted that no living gastro­ feeding in a group of low-spired Ordovician pods with paired gills are known to be ciliary pleurotomariaceans preceded the derivation feeders: "The reason for this in zeugobranchs of the Euomphalacea. is most likely to be found in the disposition of the currents within the mantle cavity—so long Diagnosis of the New Suborder Euomphalina as there are two sets of these, right and left, converging upon the mid-line, it will prove im­ The preceding account of the relationships possible for the material which they carry in between the Euomphalacea and Neomphal- GALAPAGOS RIFT LIMPET NEOMPHALUS 325

acea is concluded with the proposal of a new such an exciting find.11 Now it is to be hoped suborder for the two superfamilies, coordinate that Neomphalus, like Neopilina, will inspire in detail with the subordinal definitions of Cox others to offer alternative or modified interpre­ & Knight (1960) and Knight et al. (1960). tations. One cannot approach the subject of phylogeny without some preconceived no­ EUOMPHALINA McLean, new suborder tions, and I could hardly expect that all of those expressed here will endure. Diagnosis: Shell low-spired to discoidal, or cap-shaped; coiled shells broadly umbilicate, aperture radial; operculum (where known) ACKNOWLEDGMENTS calcified, multispiral externally, with adventi­ tious layers internally; radula rhipidoglossate; I am grateful most of all for the opportunity left ctenidium entirely bipectinate, afferent to report upon this remarkable animal, and I membrane lacking; right ctenidium and right thank those members of the committee who auricle lacking; ventricle not traversed by offered it to me. rectum; columellar muscle lateral to cephalo- Dr. J. B. Corliss of Oregon State University pedal mass. preserved the initial collection and forwarded The subordinal classification of archaeo- the material to me. Additional specimens gastropods in the Treatise (Knight et al., were sent by Dr. J. F. Grassle of Woods Hole 1960) has been both inflated (Golikov & Oceanographic Institution, Dr. M. L. Jones of Starobogatov, 1975) and deflated (Salvini- the U.S. National Museum of Natural History, Plawen, 1980)9 Dr. R. D. Turner of Harvard University, and I prefer to follow a middle ground, more or Ms. L. Morse-Porteous of Woods Hole. less equivalent to that of Cox & Knight, recog­ Serial sections were expertly prepared by nizing for now three suborders of living uni- my volunteer laboratory technician, Jo-Carol branchiate rhipidoglossates: Euomphalina, Ramsaran. Superb photograpy of whole and Trochina, and Neritina, each of which has dissected specimens was done by museum undergone major radiations that exploited the volunteer Bertram C. Draper. Scanning elec­ evolutionary potential of their very different tron micrographs of the radula were provided body plans.10 by Dr. Carole S. Hickman, University of Cali­ The addition of Neomphalus to the ranks of fornia, Berkeley (NSF Grant DEB77-14519). molluscan classification is a major milestone SEM micrographs of the juvenile shells were in malacology. New finds with as much to con­ made with the assistance of David R. Lind- tribute to our knowledge of molluscan diversi­ berg, University of California, Santa Cruz. ty and evolution are unusual events. Not since Drafts of the manuscript were read and the discovery of Neopilina has there been helpful commentary offered by Drs. Eugene an animal that could fuel so many lines of V. Coan and A. Myra Keen. Others who may speculation. Few living malacologists have have read early drafts or have helped in vari­ been as privileged as I in having free rein over ous ways through discussion and correspond-

^Salvini-Plawen's (1980: 261) suborder Vetigastropoda for superfamilies "Macluritoidea, Pleurotomarioidea, Cocculinoidea, Trochoidea, and Murchisonioidea," "defined by the dominant presence of the (posttorsional) right dorso-ventral retractor muscle as well as the right excretory organ and bilamellate ctenidia with skeletal rods," has these difficulties: Neomphalus with its skeletal rods in the ctenidium lacks the right kidney, and Cocculina has no right kidney, no skeletal rods, nor even a true ctenidium (Thiele, 1903). 10Too little is now known of the Cocculinacea, Lepetellacea and Seguenziacea to include them in this scheme. 110ver the three years that I have had Neomphalus under consideration, my conclusions about it have undergone some major changes. Progress reports have been given at meetings, which occasioned the entry of abstracts in the literature, some of the statements in which are no longer supported. The first abstract (McLean, 1979) submitted in 1978, drew no firm conclusion, although I announced at the Geological Society of America meeting in San Jose, California, on 9 April 1979 that I assigned the limpet to the suborder Macluritina as then understood. On 21 May 1979 I discussed the limpet at the Symposium on the Biology and Evolution of Mollusca at the Australian Museum, Sydney. The abstract (1980a), which was completed in April 1979, did not mention the unfound left kidney (so large and thin-walled that is was mistaken for a body cavity), but it incorrectly stated that the gonads discharge through the right kidney. In 1980 I developed my current view that the musculature of Neomphalus is the necessary consequence of its ontogeny and phylogeny. On 5 September 1980, for the Seventh International Malacological Congress in Perpignan, France, my abstract (1980b) incorrectly stated that the left kidney was vestigial. Fortunately for this novice anatomist, Drs. Fretter and Graham examined the serial section in Septem­ ber, 1980, and agreed to add their expertise to the account of the internal anatomy, resulting in the adjoining paper. The excretory and reproductive systems proved to be more advanced than I had realized, leaving Neomphalus with fewer of the archaeogastropod characters than I had originally claimed for it. 326 McLEAN ence (though not necessarily agreeing with all mariacea: Portlockiellidae, Phymatopleuridae, of my conclusions) include: R. L Batten, K. J. and Eotomariidae. Bulletin of the American Boss, G. M. Davis, J. F. Grassle, R. R. Museum of Natural History, 114: 153-246, pi. Hessler, C. S. Hickman, R. S. Houbrick, M. L. 32-42. Jones, D. R. Lindberg, R. M. Linsley, R. A. BATTEN, R. L, 1972, The ultrastructure of five common Pennsylvanian pleurotomarian gastro­ Lutz, N. J. Morris, W. A. Newman, J. Pojeta, pod species of eastern United States. American Jr., W. F. Ponder, R. Robertson, B. Runnegar, Museum Novitates, 2501: 1-34. L. v. Salvini-Plawen, R. S. Scheltema, D. E. BATTEN, R. L, 1975, The Scissur.ellidae—Are they Schindel, and R. D. Turner. neotenously derived fissurellids? American I particularly want to thank my principal re­ Museum Novitates, 2567: 1-29. viewers, Drs. Vera Fretter of the University of BATTEN, R. L, 1979, Permian gastropods from Reading, England, and Ellis Yochelson of the Perak, Malaysia. Part 2. The trochids, patellids, U.S. Geological Survey at the National Mu­ and neritids. American Museum Novitates, 2685: 1-26. seum of Natural History, Washington, D.C. BERGER, W. H., 1978, Deep-sea carbonate: Vera Fretter has provided helpful review com­ pteropod distribution and the aragonite com­ mentary throughout the entire course of this pensation depth. Deep-Sea Research, 25: 447- work. I was especially pleased that she and 452. Dr. Alastair Graham were able to add their BEU, A. G. & CLIMO, F. M., 1974, Molluscafrom a expertise to the account of the internal Recent coral community in Palliser Bay, Cook anatomy. My discussion on the Paleozoic Strait. New Zealand Journal of Marine and relationships would not have been possible Freshwater Research, 8: 307-332. without the frequent assistance of Ellis B0GGILD, O. B., 1930, The shell structure of the Yochelson, who directed me to many refer­ mollusks. Det Kongelige Danske Vidensk- abernes Selskabs Skrifter, Niende Raekke, ences and generously allowed me to cite Naturvidenskabelig Og Mathematisk Afdeling 9, some conclusions from his manuscript on the Raekke 2: 231-326, 15 pi. classification of early gastropods. BOSS, K. J. & TURNER, R. D., 1980, The giant white clam from the Galapagos Rift, Calypto- gena magnifica species novum. Malacologia, 20: 161-194. LITERATURE CITED BOUVIER, E.-L & FISCHER, H., 1902, L'Organisation et les affinites des gasteropodes ADEGOKE, O. 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vicinity of submarine thermal vents on the Gala­ APPENDIX 1: Possible Affinity of pagos rift (Crustacea: Decapoda: Brachyura). Other Extinct Superfamilies Proceedings of the Biological Society of Wash­ ington, 93: 443-472. The search for fossil predecessors to WOODWARD, M. F., 1901, The anatomy of P/euro- Neomphalus has led me to consider the rela­ tomaria bey rich ii Hilg. Quarterly Journal of Mi­ croscopical Science, 44: 215-268, pi. 13-16. tionships and possible feeding modes of YOCHELSON, E. L, 1956, Permian Gastropoda of some other extinct groups. My conclusions the southwestern United States. 1. Euom- are given in this section. phalacea, Trochonematacea, Pseudophoracea, Shell characters in the Macluritacea and Anomphalacea, Craspedostomatacea, and the Clisospiracea, as in the Euomphalacea, Platyceratacea. Bulletin of the American Mu­ exceed the limits of diversity now expressed seum of Natural History, 110:173-276, pi. 9-24. in the Trochacea. Reasons to dissociate YOCHELSON, E. L, 1960, Permian Gastropoda of these two superfamilies from the Euomphal­ the southwestern United States. 3. Bellero- acea are given here. The Oriostomatacea phontacea and Patellacea. Bulletin of the Amer­ ican Museum of Natural History, 119: 205-294, have been synonymized with the Euomphal­ pi. 46-57. acea in the body of this paper. Reasons to YOCHELSON, E. L, 1966, A reinvestigation of the synonymize the Craspedostomatacea and Middle Devonian gastropods Arctomphalus and Amberleyacea with the Trochacea are given Omphalocirrus. Norsk Polarinstitutt—ftrbok in Appendix 2. The remaining extinct super- 1965: 37-48, 2 pi. families recognized by Knight et al. (1960) YOCHELSON, E. L, 1971, A new Late Devonian and thought to be unibranchiate are the gastropod and its bearing on problems of open Pseudophoracea, Platyceratacea, Anom­ coiling and septation. Smithsonian Contributions phalacea, Microdomatacea, and Palaeotro- in Paleobiology, 3: 231-241, 2 pi. chacea. Commentary on these groups is di­ YOCHELSON, E. L, 1979a, Gastropod opercula as objects for paleobiogeographic study. In rected to the question: Do the shell characters GRAY & BOUCOT, eds., Historical Biogeog- exceed the limits now expressed in the raphy, Plate Tectonics, and the Changing Envi­ Trochacea? ronment. Oregon State University Press: 37-43. MACLURITACEA: The Ordovician genus YOCHELSON, E. L, 1979b, Early radiation of Mol- Maclurites (Fig. 16A) had an exceptionally lusca and mollusc-like groups. In HOUSE, M. R., large "hyperstrophic" shell that could only ed., The Origin of Major Invertebrate Groups. have rested on its flat base (see Banks & Systematics Association Special Volume No. 12, Johnson, 1957; Knight et al., 1960: 188). A Academic Press: 323-358, pi. 4-6. YOCHELSON, E. L, (manuscript), New data for a heavy, protruding operculum fits the aperture. revision of Paleozoic gastropod classification. Internally the operculum has two roughened YOCHELSON, E. L & JONES, C. R., 1968, areas that have been interpreted as attach­ Teiichispira, a new Early Ordovician gastropod ment scars for right and left retractor muscles; genus. [United States] Geological Survey Pro­ externally it is paucispiral with one counter­ fessional Paper, 613-B 15 p., 2 pi. clockwise volution, which provides the evi­ YOCHELSON, E. L & LINSLEY, R. M., 1972, dence that led Knight (1952) to interpret its Opercula of two gastropods from the Lilydale anatomy as dextral. The Maclurites opercu­ Limestone (Early Devonian) of Victoria, Aus­ lum is analogous to that of the Neritacea, tralia. Memoirs of the National Museum of Vic­ upon which left and right columellar muscles toria, 33: 1-14, 2 pi. YOCHELSON, E. L & WISE, O. A., 1972, A life insert, preventing it from rotating to produce a association of shell and operculum in the Early multispiral pattern. Horn-shaped opercula of a Ordovician gastropod Ceratopea unguis. Jour­ somewhat different type are known in the nal of Paleontology, 46: 681-684. macluritacean genus Teiichispira (Yochelson YONGE, C. M., 1938, Evolution of ciliary feeding in & Jones, 1968). The shell of Teiichispira is the Prosobranchia, with an account of feeding in poorly known, but Yochelson (1979a: 40) has Capulus ungaricus Journal of the Marine Bio­ concluded that it had a flattened base like that logical Association, United Kingdom, 22: 458- of Maclurites. Yochelson (in preparation) will 468. report on the recently discovered operculum YONGE, C. M., 1946, On the habits of Turritella of the macluritid genus Palliseria. communis Risso. Journal of the Marine Biologi­ cal Association, United Kingdom, 26: 377-380. Linsley (1978b, fig. 10) has depicted YONGE, C. M., 1947, The pallial organs in the Maclurites as a filter-feeding form with the aspidobranch Gastropoda and their evolution operculum loosely blocking the aperture in throughout the Mollusca. Philosophical Transac­ feeding position. Shells are heavy and the tions of the Royal Society of London, ser. B, 232: center of gravity is offset from the aperture. 443-517, 1 pi. Linsley has therefore concluded that any GALAPAGOS RIFT LIMPET NEOMPHALUS 331

A

FIG. 16. yacluritacea and Clisospiracea. A) Maclurites logani (Salter, 1859), with internal view of opercu­ lum, Ordovician (Macluritacea; Macluritidae), x0.6. B) Onychochilus physa Lindstrom, 1884, Silurian (Clisospiracea: Onychochilidae), x8.4. C) Mimospira cochleata (Lindstrom, 1884), basal and apertural views, Silurian (Clisospiracea; Clisospiridae), x3.4. A & B after Knight et al. (1960), C after Wangberg- Eriksson (1979). locomotion was by shell dragging. Maclurites place it in a family within the Macluritacea may have had the pallial configuration of because of its horn-shaped operculum, than Neomphalus, but the paired musculature that to relate it (as suggested by Yochelson & has been assumed would entail some major Wise) to the suborder Pleurotomariina. In liv­ differences from the Euomphalacea. As noted ing pleurotomariaceans (families Pleuroto- earlier, Linsley (1978c: 440) has a theory, not mariidae and Scissurellidae), the operculum as yet fully detailed, that the Macluritacea (In is multispiral. Wenz (1938: 211) placed addition to the Euomphalacea) were untorted Ceratopea in Macluritidae. and not gastropods. Yochelson (1979b: 347) The family Onychochilidae, included by has mentioned the possibility that the small Knight et al. (1960) in the Macluritacea, is Cambrian Pelagiella could be ancestral to the here transferred to the Clisospiracea, as dis­ Macluritacea, though he now (manuscript) cussed under the following heading. favors retention of Macluritacea as a gastro­ CLISOSPIRACEA: The Clisospiridae (Fig. pod lineage apart from Euomphalacea, rather 16C) and Onychochilidae (Fig. 16B), both than their predecessors, as implied by Knight moderately to extremely high-spired and ap­ et al. (1960). parently sinistral, are here united in the super- The Macluritidae are now limited to genera family Clisospiracea. Although Knight (1952) with horn-shaped opercula; these genera are included Clisospira among the supposedly known only from the Ordovician. Omphalocir- hypertrophic genera related to Maclurites, rus was transferred to the Euomphalacea by this position was reversed by Knight et al. Yochelson (1966) and Lecanospira (Fig. 15B) (1960), who interpreted Clisospira as sinistral. to the Pleurotomariacea (Yochelson manu­ The Clisospiracea, then containing only script). The Ordovician Ceratapea is another Clisospiridae, were grouped among those genus with a horn-shaped operculum of yet superfamilies of "doubtful subordinal posi­ another kind. Its poorly known shell was first tion." The Onychochilidae were regarded as associated with its well-known operculum by dextral-hyperstrophic and were included in Yochelson & Wise (1972). The shell is the Macluritacea, apparently in the belief that orthostrophic, thereby differing from other there were transitional forms leading to macluritids, but I would be more inclined to Maclurites. More recently. Horny (1964), Peel 332 McLEAN

(1975b), and Wangberg-Erikkson (1979) too few progalerine specimens known to en­ have found transitional forms between the able any firm conclusions. Onychochilidae and the Clisospiridae. This This analysis, however, is complicated by led again to the assumption that clisospirids the fact that some Mimospira species have were hyperstrophic like the onychochilids and heterostrophic (not hyperstrophic) proto- therefore to the assignment of both families to conchs (Peel, 1975b: 1528): "The protoconch the Macluritacea. However, because opercula is an open-coiled half whorl which, by way of a are unknown in both families, there is no di­ perpendicular change in direction of the axis rect evidence of hyperstrophy, and the entire of coiling from horizontal to vertical, assumes assumption is open to question. the hyperstrophic form of the teleconch." Be­ Whether the two families were sinistral or cause heterostrophic protoconchs are un­ dextral-hyperstrophic, they differ from known in Recent archaeogastropods, I offer Macluritacea and Euomphalacea in having no further speculation. Linsley (1977:204, fig. tangential rather than radial apertures. 7; 1978b: 201, fig. 9; 1978c, figs. 3, 12) has Onychochilids and clisospirids would have depicted Onychochilus (Fig. 16B) as carrying been able to clamp to the substratum and the shell with the spire directed anteriorly over some should have been capable of more ef­ the head of the animal. Such an unorthodox fective locomotion than that of a "shell drag- interpretation presumably is explained in his ger." The ontogenetic change in orientation, theory (1978c) that the entire group compris­ which would be required in euomphalacean ing the Macluritacea and Euomphalacea was and macluritacean development, was not a untorted. The Onychochilidae appeared in the component in onychochilid and clisospirid de­ Upper Cambrian and thus are among the velopment. The tangential rather than radial earliest known gastropods. A convincing ex­ aperture plus the lack of the appropriate planation of their form and function would be opercula is sufficient reason to exclude them of great importance to an understanding of from either the Macluritacea or Euomphal­ gastropod phytogeny. acea. PSEUDOPHORACEA: Linsley et al. (1978) The Clisospiridae, exemplified by Mimo- have discussed the life habits of pseudo- spira (Fig. 16C), have moderately high-spired phorid genera (Fig. 17A) that have a periph­ shells with smooth, concave bases. The only eral frill, an extension of the base of the shell possible interpretation of the relation of such a serving to raise the position of the aperture shell to the substratum is that it attached, above the substratum. As in the Euomphal­ limpet-like, to hard surfaces. Hyperstrophy by acea the coiling axis is perpendicular to the definition means that the internal anatomy is substratum, but the lip growth is prosocline dextral, with water currents flowing left to and the aperture is tangential, so that the right, despite the sinistrality of the shell. base of the shell is shielded on all sides. They Dextral anatomy is entirely possible within a concluded that the frill-bearing pseudophorids high-spired sinistrally coiled shell like the could have lived on a firm, but not hard, sub­ ampullariid Lanistes (see Cox, 1960:110, fig. stratum, much as in the extant deposit-feed­ 67), in which the plane of the aperture is near­ ing Xenophoridae. Retention of spiral sculp­ ly parallel to the axis of coiling, but it is not ture on the base of the Permian Sallya (Fig. possible in a shell form in which the axis of 17A) precludes the limpet-like mode of the liv­ coiling is perpendicular to the plane of the ing calyptraeid Trochita, in which the entire aperture (Fig. 16C). The left ctenidium under base of the shell is smooth. The absence of such an impossible condition would be forced inhalant access in the shell is no hindrance to to curve backwards around the columella. filter-feeding limpets on hard substrates, but Thus the Clisospiridae could only have been the example of Turritella, as well as that sinistral in both shell and anatomy. If there is a hypothesized for the Euomphalacea, sug­ transition between the Clisospiridae and the gests that filter feeders on soft substrates Onychochilidae, as has been proposed by would not provide a tentlike shield over the Horny, Peel and Wangberg-Erikkson, then it head. I therefore think that the best hypothe­ follows that the Onychochilidae were also sis is that pseudophorids were deposit feed­ anatomically sinistral. The Devonian Pro- ers. Although there are no living trochaceans galerinae (see footnote 3) were regarded by with a peripheral frill, there are deposit-feed­ Knight et al. (1960) as dextral clisospirids. It is ing trochaceans. I can think of no argument possible that there were dextral as well as that would preclude the Pseudophoracea sinistral clisospiraceans, although there are from having the trochacean pallial complex.