Novitates PUBLISHED by the AMERICAN MUSEUM of NATURAL HISTORY CENTRAL PARK WEST at 79TH STREET, NEW YORK, N.Y

Home , Fordilla, Ligament (bivalve), Limopsidae, Resilifer

AMERICAN MUSEUM Novitates PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, N.Y. 10024 Number 2875, pp. 1-1 1, figs. 1-10 April 23, 1987

Iteration of Ligament Structures in Pteriomorphian Bivalves

NORMAN D. NEWELL1 AND DONALD W. BOYD2

ABSTRACT Pteriomorphia, the largest subclass of the Bi- For example, one complex structure (duplivin- valvia, includes such highly successful extant cular) is replaced geochronologically by a different, groups as the marine mussels, pearl shells, oysters, and simpler structure (alivincular) in families of and scallops. Shells ofdifferent groups display sur- the Arcacea, Anomiacea, Aviculopectinacea, and prisingly diverse structural types of ligaments (di- Pteriacea. "Transitional" (new term), multivin- agnostic traces ofwhich are recognizable in fossils cular, and duplivincular ligaments all occur in the as old as the Early Cambrian) and the ligaments superfamily Pteriacea. Even where the geological have figured prominently in pteriomorph classi- succession of ligaments might be interpreted as fications. indicating an ancestral-descendant relationship, Within the context ofgeneral morphology, sev- the different grades of ligaments do not form gra- en or eight ligament grades have been used to char- dational morphoclines. acterize families of pteriomorphs. But the same, So this review is intended only to call attention or very similar, ligaments appear repeatedly as to taxonomic and phylogenetic difficulties in eval- evolutionary novelties in separate taxonomic uating ligament grades; we are withholding con- groups without evident phyletic origins or marked clusions about classification until later. adaptive significance. Even their phylogenetic polarity is frequently in doubt.

INTRODUCTION The bivalve subclass Pteriomorphia is a Treatise on Invertebrate Paleontology (New- major cluster of46 families, according to the ell, 1969). Its great diversity may well be an

' Curator Emeritus, American Museum of Natural History. 2 Research Associate, American Museum of Natural History; Professor of Geology, University of Wyoming, Laramie, Wyoming.

conservative and stable for tens to hundreds ofmillions ofyears, giving credibility to much of the accepted family arrangement. The stratigraphic sequence offossils, together with development, skeletal morphology, anato- B my, and life modes of living representatives, A all contribute to ideas about phylogeny and Fig. 1. Cross sections showing the two liga- classification. There remain, however, many ment layers: lamellar (black) and fibrous (ruled). differences, partly semantic, as to limits and Hinge axis is indicated by cross. Stipple pattern represents shell. A. Primitive; B. parivincular. names of higher categories (e.g., compare (From Trueman, 1 951) Newell, 1969, and Waller, 1978). These are being smoothed out with time. One ofthe most interesting characteristics artifact of its classificatory history. As so de- of this and some other groups of Bivalvia is fined it spans the entire Phanerozoic and a tendency toward evolutionary repetition of comprises about one-fourth ofall the families shell characters, which is probably attribut- ofbivalve molluscs, including such successful able to the limited ways in which structural groups as the arks, scallops, pearl shells, oys- modifications have been genetically con- ters, and marine mussels. They include strained. Diverse structural types of the monomyarian, anisomyarian, and isomyari- pteriomorph ligament provide examples of an forms with diverse ligament structures. parallel repetition of a morphological char- Early attempts at classification stressed sin- acter in separate taxonomic branches. gle anatomical characters, such as gill types, As with other bivalves, the ligament con- that leave no indication on the shells (e.g., sists of two parts. One of these is an upper, Isofilibranchia), or the reduction or absence elongate, lamellar band usually under tension of the anterior adductor muscle repeated in when the valves are closed. The other is an separate branches. But these arrangements underlying fibrous layer subject to compres- have fallen into disfavor with increasing sion when the valves are closed (fig. 1). The knowledge ofthe fossil record ofthe Bivalvia latter is stiffened by microscopic needles of (Newell, 1965, 1969). aragonite arranged at right angles to the inner The shell characters that delimit many of growing surface where both components are the conventional family groups have been secreted by the mantle.

7,--

b c

O lOmm I a @ + f 0 1 2mm a I~~~~~~~~~~~~~ Fig. 2. Transitional ligament illustrated by the arcoid Lunarca ovalis. Recent, coastal New Jersey. Serial sections showing median split of fibrous and lamellar layers and added secondary lamellar band below the beaks. Note that the split begins well in front ofthe posterior end ofthe hinge. Lamellar layers in black; fibrous layer lined; mantle stippled. Based on several specimens. 1987 NEWELL AND BOYD: LIGAMENT STRUCTURES IN PTERIOMORPHIA 3

LIGAMENT GRADES

AXIS AXIS >2 ...... AUVINCULAR-EX(TERNAL ( ALVINCULAR-INTERNAL

AXIS ,.' AXIS N I DUPLVINCULAR-AMPHIDETIC .\ X MULT VINCULAR AXI'IS *

TRANSITIONAL

AXIS * , 9SAXIS

AXIS- I - DUPLVINCULAR-OPISTHODETIC S ~~~~~~~~PARMVNCULAR

PRIMITIVE Fig. 3. Ligament grades with possible but uncertain paths of change, some of which could have resulted from neotenous reversal. The phyletic polarity is usually uncertain. S, secondary lamellar band. The alivincular and duplivincular figures at the top are amphidetic; the others are opisthodetic, black, lamellar, ruled, fibrous.

The fibrous layer is weak to tensional stresses and in some forms tends to split as interumbonal growth spreads the two valves apart. Hence, the functional part of the fibrous layer lies below the hinge axis. In this situation expansion carries the obsolete part ofthe ligament above the hinge axis. In some taxa the fracture is repaired by a secondary plug, or band, of lamellar conchiolin which maintains the juncture ofthe two valves (fig. 2). The ligament usually lies behind the beaks, the opisthodetic condition. Or it may extend both in front of, and behind, the beaks where it is said to be amphidetic (fig. 3). It seems that the amphidetic hinge appeared late in bivalve history (mid-Paleozoic) and there- fore was probably "derived" from the more ancient and evidently more primitive opisthodetic condition. A stable trait in some families is the pos- Fig. 4. Hinge axis in relation to internal and session of flat or slightly concave cardinal external ligament. A. Internal ligament without areas (fig. 4B) to which the ligament is at- hinge plates or cardinal areas; B. external ligament tached. These result from the inward and with cardinal areas and hinge plate; UC indicates downward migration of the hinge axis as umbonal cavity. Stipple area represents shell; black growth expands the valves between the um- circle, hinge axis. 4 AMERICAN MUSEUM NOVITATES NO. 2875

B C

D h".,,

Fig. 5. Growth-line traces of transitional ligament in several families of Pteriomorphia. The scale bars represent 5 mm. A. Pterioid (pergamidiid), Eurydesmaplayfordi Dickins. Permian, eastern Australia, Austral. Bur. Mineral. Resources no. 2232. B. Cyrtodont, Cypricardinia sp., Permian, West Texas, Am. Mus. Nat. Hist. no. 42886. C. Parallelodont, probably a n. gen., n. sp. Transitional ligament limited to posterior part of cardinal area, duplivincular on anterior part. Permian, West Texas, Am. Mus. Nat. Hist. no. 42887. D. Arcoid, Lunarca ovalis. Recent, Georgia. Am. Mus. Nat. Hist. no. M28579. Arrow points to groove of the secondary lamellar band.

bones. The cardinal areas are thus external Here we consider eight well-defined types, and they converge downward from the dorsal or grades, of ligaments. They may be desig- margins to the hinge line at about 15 up to nated primitive, transitional, parivincular, 450. opisthodetic-duplivincular, amphidetic-du- 1987 NEWELL AND BOYD: LIGAMENT STRUCTURES IN PTERIOMORPHIA 5 plivincular, external-alivincular, internal- alivincular, and multivincular (fig. 3). In conventional usage many of the pteriomorph families display only one grade of ligament but there are some with more than one lig- B ament type. Ambiguity thus introduced suggests a need to reevaluate the taxonomic significance of the several ligament grades in a general morphological and historical context. Our purpose here is to report some obser- A vations and ideas resulting from our con- tinuing studies of bivalves that lived in the C seas around the time ofthe great Late Perm- Fig. 6. Sections ofmodern arcacean ligaments. ian and Early Triassic extinction crisis. It is A. Transitional grade in Lunarca ovalis (=pexata) a cursory review in which we postpone a con- Say. Dot indicates location of hinge axis. Fibrous sideration of pteriomorphian systematics. layer (ruptured), below; lamellar above. B, C. Du- We are indebted to E. R. Trueman and plivincular ligament of Scapharca transversa. Thomas R. Waller for reading our manu- Three-dimensional view (above) shows location script and making useful suggestions. ofsection (below). Two bands oflamellar ligament are separated by split and inoperable fibrous layers THE PRIMITIvE LIGAMENT (after Newell, 1942). Stipple pattern represents mantle; black, the shell. The most ancient bivalves, Fordilla and Pojetaia ofthe Lower Cambrian, had the sim- plest ligament remindful of modern Mytilus cyrtodonts, arcids, and parallelodonts (fig. 5). or Malletia (Runnegar and Bentley, 1983) A living example is the arcid genus Lunarca (figs. IA, 3, primitive). The ligament in Myt- in which the structure is that of a juvenile ilus, however, is reinforced below by shell duplivincular ligament possessing a single "pseudonymphs." dorsal band rather than multiple lamellar Runnegar and Bentley and some others bands (fig. SD). It may have been a simpler (e.g., Morris, 1979) referred to this ligament precursor of the duplivincular ligament (see as parivincular (figs. 1B, 3), but we prefer to below), but it could also conceivably be pae- reserve that term in the conventional sense dogenetic, derived from that structure by for the more advanced C-spring ligament of neoteny. The phyletic polarity is uncertain the heterodonts, unionids, and trigoniaceans and we suspect that both conditions may ex- (fig. 1B). The primitive ligament has also been ist. called primary by Owen et al. (1953). The transitional ligament is usually at- In the primitive ligament, lamellar and fi- tached to the outer areas ofhinge plates (figs. brous layers form an elongate ribbon between 4B, 5, 6A). In addition to the single lamellar the dorsal margins of the valves (Trueman, band at the dorsal margin of each cardinal 1950). The lamellar part extends farther to- area a secondary band is inserted below the ward the rear than the fibrous part in an over- beaks near the hinge line where the fibrous lapping relationship that keeps them in con- layer splits (figs. 2e, f, 5D). This seems to tact with the mantle so that both increase in compensate for the loss of function of the length as the shell grows. As the ligament split portion ofthe fibrous band in the inter- splits anteriorly the gap in the fibrous part is umbonal area. reinforced by a secondary band of lamellar conchiolin. THE DUPLIVINCULAR LIGAMENT THE TRANSITIONAL LIGAMENT The duplivincular ligament (Newell, 1942, The transitional ligament may be opis- p. 29) is composed of a series of alternating thodetic or amphidetic. It occurs in several bands of lamellar and fibrous conchiolin ex- pteriomorphian groups, e.g., pergamidiids, tending obliquely along each cardinal area 6 AMERICAN MUSEUM NOVITATES NO. 2875

CZ- aD~~~~~~~~~~Aw~

Fig. 7. Duplivincular ligament in diverse pteriomorph families. Scale bars represent 5 mm. A. An Anomiacean, Permanomia texana Newell and Boyd, with opisthodetic-duplivincular hinge. Permian, West Texas, Am. Mus. Nat. Hist. no. 28937. B. Arcid, Scapharca transversa (Say) with amphidetic- duplivincular ligament area. Recent, Florida, Am. Mus. Nat. Hist. no. M28708. C. Cyrtodont, Vanux- emia gibbosa Ulrich, opisthodetic-duplivincular ligament. Middle Ordovician, Tennessee, U.S. Natl. Mus. no. 46942. The very fine ligament grooves simulate growth lines but, unlike growth lines, the last 1987 NEWELL AND BOYD: LIGAMENT STRUCTURES IN PTERIOMORPHIA 7

(figs. 6B, C, 7). Each lamellar band is in- of the shell. As an alternative, he suggested serted in a narrow groove that extends from that the grooves represent successive inser- below the beaks to the hinge margin. The tions ofa single, ventrally migrating ligament bands resemble replications of the dorsal la- band. If so, the situation would be unlike any mellar band of the transitional ligament. living bivalve known to us. The duplivincular ligament is commonly opisthodetic in juveniles and most adults but THE ALIVINCULAR LIGAMENT also it may be symmetrically amphidetic or become amphidetic during growth, as in This ligament is characteristically amphi- Scapharca transversa (fig. 7B). The grooves detic, rarely opisthodetic. The compressional in the amphidetic varieties form an inverted part is a- more or less triangular pad, the re- chevron pattern with the apex under the silium, segregated in a triangular depression, beaks. The duplivincular ligament character- the resilifer, between anterior and posterior izes several families in the Cyrtodontacea, areas of lamellar ligament extending along Pteriacea, Pectinacea, Anomiacea, and Ar- the length ofthe hinge axis (fig. 8A, B, D, E). cacea. Some have elongated resilifers in which the Recognition ofthe duplivincular condition base ofthe triangle greatly exceeds its height. in extinct taxa poses no problem where the Rarely, the resilifer is very broad, asymmet- ligament area exhibits coarse grooves that in- ric, and marked with longitudinal grooves tersect the hinge margin at a high angle (fig. that suggest seasonal growth interruptions (fig. 7A, B, D, and E). At the other end of the 8A). duplivincular spectrum, however, the angle The alivincular ligament may be attached ofintersection may be so low that all but the to the cardinal areas of hinge plates, as with one or two last-formed grooves extend all the Aviculopectinidae and Pteriidae. This is along the length of the hinge and appear to called the external alivincular ligament (figs. be parallel with its ventral margin (fig. 7C, 3, 4B, 8A, B, E). F). This has led Dickins (1983), to conclude Or, the alivincular ligament may be essen- that the relationship of grooves to margin is tially internal, in which case true cardinal not significant. We, however, believe that areas are lacking (figs. 3, 4A, 8D). The inter- confusion arises only where the grooves are nal resilium of the Pectinidae, for example, very fine and the preservation of fossils is differs from the external resilium of other imperfect. Pteriomorphia. The central part ofthe former Discrimination between certain duplivin- lacks aragonite needles (fig. 9B) and the cal- cular, transitional (fig. 5), and elongate ali- cified lateral portions are rigid and of uncer- vincular (fig. 8A, B) ligament areas can be tain function (Newell, 1937; Trueman, 1953; difficult. Pojeta (1978, p. 236) encountered a Waller, 1976). They may be evolutionary rel- problem in dealing with Ordovician cyrto- icts of the fibrous resilium. dontids. Noting that the parallel grooves on The curious Upper Paleozoic aviculopec- his shells apparently did not intersect the tinacean genus Euchondria (Euchondriidae) hinge margin, he concluded that the ligament has an external alivincular ligament. The adult was not duplivincular. He also rejected the hinge retains juvenile pseudotaxodont teeth growth-line explanation because the grooves (fig. 1OA) similar to those of many immature on the ligament area seemed to lack lateral pteriomorphs (fig. lOB). Some modern pec- continuity with growth lines on adjacent parts tinids also retain these neotenous structures formed grooves are progressively shorter and they intersect the hinge margin. D. Cyrtodont with opisthodetic-duplivincular ligament grooves, Ptychodesma knappianum Hall and Whitfield. Middle Devo- nian, New York State, Am. Mus. Nat. Hist. no. 361888. E. Myalinid, Septimyalina perattenuata (Meek and Hayden). Ligament opisthodetic-duplivincular. Upper Pennsylvanian, Kansas, Am. Mus. Nat. Hist. no. 42888. F. Myalinid, Selenimyalina meliniformis Newell. Ligament opisthodetic-duplivincular. The numerous ligament grooves are comparatively fine. Upper Pennsylvanian, Kansas, Am. Mus. Nat. Hist. no. 42889. 8 AMERICAN MUSEUM NOVITATES NO. 2875

Fig. 8. Diverse pectinoid ligaments. Scale bars represent 5 mm. A. Deltopectinid, Deltopecten sp. Interpreted by comparison with B, below, as external-amphidetic. The very elongate resilifer is marked by regular, probably seasonal, growth lines. Permian, South Kennedy, Queensland, Australia, Am. Mus. Nat. Hist. no. 42890. B. Strebloptenid, Streblopteria sp., similar to fig. 8A but lacking conspicuous growth lines. Permian, West Texas, Am. Mus. Nat. Hist. no. 42891. C. Pterinopectinid with an amphidetic-duplivincular ligament. Pterinopectinella sp. Permian, West Texas, U.S. Natl. Mus. no. 388871. D. Entoliid, Entolium sp. Illustrating internal-alivincular ligament. Permian, Texas, U.S. Natl. Mus. no. 388870. E. Aviculopectinid, Etheripecten vanvleeti (Beede). External-alivincular ligament. Permian, Texas, Am. Mus. Nat. Hist. no. 42893. that continue to be functional well into ma- cussed Euchondria he mistook the gaps be- turity (fig. lOC). In 1937 when Newell dis- tween denticles for multivincular resilifers. 1987 NEWELL AND BOYD: LIGAMENT STRUCTURES IN PTERIOMORPHIA 9

Fig. 10. Pseudotaxodont hinges characteristic of the early development of some pteriomorphi- ans. Scale bars represent 0.5 mm. A. Neotenous hinge in Euchondria levicula Newell, a small adult aviculopectinacean with an external ligament. Middle Pennsylvanian near St. Louis, Mo., Univ. Kansas no. 58805; B. external ligament injuvenile Fig. 9. Internal alivincular ligament ofChlam- Pteria staminensis, Recent, after Bernard (1896); ys islandica Miiller, modern pectinid. A. Trans- C. internal ligament of Pecten magellanicus, Re- verse section of lamellar band near posterior end cent, after Jackson (1890). of the hinge (stipple area represents mantle). B. Transverse section at hinge midlength of uncal- cified compressional ligament (center), flanked by relict nonfunctional fibrous ligament pads. (Cross Morris (1979, fig. 8) represents hinge axis. Black indicates shell. After support this hypothesis. Newell, 1937.) thought that the multivincular ligament might be derived from the duplivincular structure but we do not know of any evidence for this. In the Permian Period the multivincular THE MULTIVINCULAR LIGAMENT ligament suddenly appeared in a population This ligament possesses multiple resilifers ofthe Australian ambonychiid genus Atomo- of the external alivincular type and it is usu- desma (Aphanaia) which had a very different ally opisthodetic. Among living bivalves (fig. (transitional) ligament (Browne and Newell, 3) the multivincular ligament is characteristic 1966; Kauffman and Runnegar, 1975; Newell of the Isognomonidae (Pteriacea). It is also and Boyd, 1978; Dickins, 1983). The Perm- the mode ofseveral extinct families including ian multivincular variety was given the ge- the very successful Mesozoic Inoceramidae neric name Permoceramus by Waterhouse and some aberrant oysters which have been (1970). Later Muromtseva (1979) found the given the generic name Pernostrea (Stenzel, same association of Permoceramus and 1971, p. 974). The Noetiidae (Arcacea) pos- Atomodesma in northeastern Siberia. Except sess superficially similar multiple resilia but for the ligament, the shells of the two genera they are unique in being formed of lamellar are virtually identical and they are very like rather than fibrous bands. the Mesozoic Inoceramidae. Sequential development of the multivin- Everyone who has studied the geological cular ligament in an individual suggests that occurrences of Permoceramus has regarded it could have been derived phyletically from it as marking the intrapopulation origin of the external-alivincular ligament by serial the multivincular pteriacean family Inoce- repetition. However, as well as we can now ramidae which otherwise is not known prior ascertain the stratigraphic succession does not to the Jurassic a hundred million years later. 10 AMERICAN MUSEUM NOVITATES NO. 2875

Why was Permoceramus so short-lived as duplivincular ligaments and probably a small, compared with the later inocerami? weak foot, had thin, flattened shells shaped Because of this exceptional Permian oc- very much like living scallops. By analogy currence Kauffman and Runnegar (1975) have they were probably active members of the suggested that the multivincular family Ino- epifauna. ceramidae may logically be extended to in- The Mytilidae, Ambonychiidae, Myalini- clude not only Permoceramus, but also the dae, and Inoceramidae probably were all associated Atomodesma with its transitional adapted to similar modes of life-i.e., sed- ligament. entary and byssate. Apart from the fact that Could Permoceramus simply be an unsuc- they were all opisthodetic, they had different cessful variant that was repeated again in the ligament styles-what we are calling primi- Jurassic as Inoceramus? Ifthey are not closely tive, transitional, duplivincular, and multi- related this would be a most remarkable ex- vincular, respectively. There is little, if any, ample of nearly identical iteration because correlation in these families between shell Permoceramus is morphologically an ino- form and ligamental grade. The ligaments ceramid in all known details. Or did Per- alone do not tell us about life modes. moceramus retire to an unknown haven during the Permo-Triassic environmental crisis until its chance to spread throughout the world CONCLUSION as Inoceramus in the later Mesozoic? Although the several distinctive kinds of pteriomorphian ligament patterns have been ADAPTIVE SIGNIFICANCE OF given high priority in taxonomic discrimi- LIGAMENT PATTERNS nation and some appear to have been stable The adaptive significance of ligament type for long geological intervals, they are not is not clear. In a series ofexperiments in 1950- themselves reliable guides to relationships at 1969, Trueman studied the relative efficiency high taxonomic levels. The various ligament of several kinds of ligaments in living bi- grades are basically the same in construction valves. This was done by measuring stress and function but they differ in patterns that differences between opening and closing mo- have uncertain interrelationships. The high ments of the valves as follows (Trueman, frequency of convergence of ligament grades 1964). in separate taxonomic categories indicates a need for a thorough revision of morphology Ratio of Ligament Resistance to and taxonomy ofthe subclass beginning pref- Muscle Adduction (in Grams) erably at low taxonomic levels. Arcacea (duplivincular) 83/105, 79% Mytilacea (primitive) 660/780, 84% Ostreacea (alivincular-external) 370/425, 87% LITERATURE CITED Pectinacea (alivincular-internal) 160/167, 96% Bernard, Felix Evidently there is some relationship be- 1895-97. Sur la development et la morphologie tween ligamental resistance and mode of life de la coquille chez les lamellibranches. of the various species, but it is not marked. Soc. Geol. France, Bull. ser. 3, 24 (1896): Shallow burrowers among living pterio- 54-82, 412-449. morphs (e.g., arcoids, fig. 2) have relatively Browne, I. A., and Norman D. Newell weak ligaments which are aided in opening 1966. The genus Aphanaia Koninck, 1877- the valves by a strong foot (Thomas, 1978), Permian representative of the Inoce- have relatively stronger and ramidae. Am. Mus. Novitates, 2252:1- while swimmers 10. more efficient ligaments. Among these, the Dickins, J. M. Pectinidae have internal alivincular liga- 1983. Posidoniella, Atomodesma, the origin of ments but the Limidae, with external alivin- the Eurydesmidae, and the develop- cular ligaments, are also swimmers. ment of the pelecypod ligament. Bur. Contrasted with the more convex arca- Mineral Res. Australia, Bull., 217:59- ceans, the Paleozoic Pterinopectinidae, with 65. 1987 NEWELL AND BOYD: LIGAMENT STRUCTURES IN PTERIOMORPHIA I1I

Jackson, R. T. lum Mollusca. U.S. Geol. Survey ProfJ 1890. Phylogeny ofthe pelecypoda-The Av- Pap. 568:1-88. iculidae and their allies. Boston Soc. Nat. Runnegar, Bruce, and Christopher Bentley Hist., Mem., 4:277-400. 1983. Ancestry, ecology and affinities of the Kauffman, E. G., and Bruce Runnegar Australian Early Cambrian bivalve Po- 1975. Atomodesma (Bivalvia) and the Perm- jetaia runnegari Jell. J. Paleontol., 57: ian species of the United States. J. Pa- 73-92. leontol., 49:23-51. Stenzel, H. B. Morris, N. J. 1971. Oysters, In R. C. Moore (ed.), Treatise 1979. On the origin of the Bivalvia. In M. R. on invertebrate paleontology, Part N, House (ed.), The origins of major in- Mollusca 6, vol. 3, pp. 953-1224. Law- vertebrate groups. New York: Academic rence: Geol. Soc. Am. and Univ. of Press, pp. 381-413. Kansas. Muromtseva, V. A. 1979. Representative of the Inoceramidae in Thomas, R. D. K. the Upper Permian deposits ofthe Ver- 1978. Limits to opportunism in the evolution khoyan'e region. Upper Paleozoic and of the Arcoida (Bivalvia). Phil. Trans. Mesozoic: Islands and coast ofthe Arc- Roy. Soc. London B, 284:335-344. tic Ocean USSR. NIIGA, Geol. Minis- Trueman, E. R. try, USSR:34-37. 1950. Observations on the ligament ofMytilus Newell, Norman D. edulis. Q. J. Microscop. Sci., 91:225- 1937 (1938). Late Paleozoic pelecypods: Pec- 235. tinacea. Kansas Geol. Surv. Publ., 10(1): 1951. The structure, development, and oper- 1-123. ation of the hinge ligament of Ostrea 1942. Late Paleozoic pelecypods: Mytilacea. edulis. Q. J. Microscop. Sci., 92:129- Kansas Geol. Surv. Publ., 10(2): 1-115. 140. 1965. Classification ofthe Bivalvia. Am. Mus. 1953. The ligament ofPecten. Q. J. Microscop. Novitates, 2206:1-25. Sci., 94:193-202. 1969. Classification of Bivalvia. In R. C. 1964. Adaptive morphology in paleoecologi- Moore (ed.), Treatise on invertebratepa- cal interpretation. In John Imbrie and leontology, Part N, Mollusca 6, vol. 1, Norman D. Newell (eds.), Approaches pp. 205-218. Lawrence: Geol. Soc. Am. to paleoecology, pp. 45-70. New York: and Univ. of Kansas. Wiley. Newell, Norman D., and Donald W. Boyd 1969. Ligament. In R. C. Moore, (ed.), Trea- 1978. A palaeontologist's view ofbivalve phy- tise on invertebrate paleontology, Part logeny. Phil. Trans. Roy. Soc. London N, Bivalvia, vol. 1, pp. 58-64. Geol. Soc. B, 284:203-215. Am. and Univ. of Kansas. Oliver, P. Graham Waller, T. R. 1981. The functional morphology and evo- 1976. The development ofthe larval and early lution of recent Limopsidae (Bivalvia, postlarval shell of the bay scallop, Ar- Arcoidea). Malacologia, 21:61-93. gopecten irradians. Bull. Am. Malacol. Owen, G., E. R. Trueman, and C. M. Yorge Union for 1976:46. 1953. The ligament in the Lamellibranchia. 1978. Morphology, morphoclines and a new Nature, 171:73-75. classification ofthe Pteriomorphia. Phil. Pojeta, John Trans. Roy. Soc. London B, 284:345- 1978. The origin and early taxonomic diver- 364. sification of pelecypods. Phil. Trans. Waterhouse, J. B. Roy. Soc. London. B 284:225-243. 1970. Permoceramus, a new inoceramid bi- Pojeta, John, and Bruce Runnegar valve from the Permian ofeastern Aus- 1976. The paleontology of rostroconch mol- tralia. N.Z. J. Geol. Geophys., 13:760- lusks and the early history of the Phy- 766. Recent issues of the Novitates may be purchased from the Museum. Lists of back issues of the Novitates, Bulletin, and Anthropological Papers published during the last five years are available free of charge. Address orders to: American Museum of Natural History Library, Department D, Central Park West at 79th St., New York, New York 10024.