The Molluscs of the Great African Lakes. 187

THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 187

The Molluscs of the Great African Lakes. IV. Nassopsis and Bythoceras. By .1. E. §. Wloorc.

With PIHI.CS 20 ami 2].

AMONG the numerous forms of molluscs that appear to he peculiar to Lake Tanganyika at the present time, there are three very marked genera—Paramelania, Nassopsis, and Bythoceras, which, owing to the substantial similarity existing between their shells, might, and have hitherto been regarded as closely associated forms. The first of these to be discovered, Nassopsis, was originally classed by S. P. Wood- ward1 among the Melanias, and as a member of the sub-genus Melanella, his determination being made from some empty- shells which had been brought from Tanganyika by Captain Speke, during Burton's celebrated journey to that lake. The second genus, Paramelania, was formed by Smith,2 in 1881, to include a Somewhat similar form of empty shell which had been brought from the same locality by the missionaries; and to distinguish the members of this genus from the original Melanella nassa of Woodward, Smith3 placed the latter form in the new genus of Nassopsis. In 1896 I4 dredged in the deep water of Tanganyika ' ' Zool. Soe. Proc.,' 1857. • Ibid., 1881, p. 559. 3 ' Ann. Mag. Nat. Hist.,' 1890, vol. vi, p. 93. 4 The genus was first named in my paper in the ' Proc. Roy. Soc.,' vol. Ixii, p. 451. The full diagnosis is contained in the 'Proc. Mall. Soc.,' 1898. 188 J. B. S. MOORE. another form, which although it presents, when adult, the peculiar horns above and below the mouth represented in PI. 21, fig. 3, still bears a most remarkable resemblance to both Paramelania and Nassopsis. For this new form, and before I was acquainted with more than the shells of any of the three, I proposed the third generic name, Bythoceras. Now, without further knowledge, anyone judging from the appearance of the shells of these three different genera would certainly regard them as heing in all probability closely related to one another. Therefore, holding the opinion that the conchological method of determining molluscan relationships is utterly unsound, it is with the greatest satisfaction that I am here enabled, by a detailed account of the anatomy of Nassopsis and Bythoceras, to show that even in their wider phylogenetic sense these genera bear no relation to each other. The characters of both, however, are of far greater importance to morphologists than that of affording them material wherewith to exhibit the futility of attempting to determine the nature of molluscan affinities from empty shells. The anatomical features of Nassopsis are in many ways quite unlike those of any forms hitherto described; and I shall immediately make it clear that this form presents us with an Archi-tsenioglossa of an entirely new type. This being so, when we consider the unquestionably vast antiquity of the lake in which Nassopsis lives, and the fact that its shell, along with numerous others still living in Tanganyika, is specifically indistinguishable from forms which once abounded in the old Jurassic seas,1 it will be readily accorded by those interested in prosobranchiate morphology that in Nassopsis we are presented with an animal which in the future will probably constitute one of the most important prosobranchiate archetypes of which we are in search. 1 " On the Hypothesis that Lake Tanganyika represents an old Jurassic Sea," 'Quart. Journ. Micr. Sci.,' vol. 41, 1898, p. 303. THE MOLLUSCS OF THE GRRAT AFRICAN LAKMS. 189

(A) Nassopsis nassa. During life this mollusc inhabits the surface rocks of Tan- ganyika, and its shells are always richly encrusted with the green algae which clothe the rocks for a considerable depth. It is sluggish, and appears to browse within a very limited area, like the Patellas of the ocean beach. The foot is broad, somewhat pigmented, and quite white in places ; the snout is broad, black, and wrinkled, not protrusible, but retractile. The tentacles are short and black, and the eyes are not carried

int.

' Crist.S.S. a.crtsts.s.

r b.ci.

S.C. FIG. I.—Stomach of Nassopsis nassa opened from above, showing relation of crystalline style-sac to main chamber of stomach, Si. Bristle passed from aperture of oesophagus along its interior, b 2. Bristle passed from aperture of intestine to the cut end of same. Crist, s. s. Crystalline style-sac, a. crist. s. s. Opening between stomach and crystalline style-sac, b.d. Aperture of digestive gland. S. c. Spiral caecum. 190 J. B. S. MO0EE. on the tentacles themselves, but on secondary papillae at their posterior bases (PI. 20, figs. 5, 12, 6). There is a well-developed mucous gland in the mantle cavity (PI. 20, fig. 5, m.g.), and the animal, unlike the genus Spekia, is viviparous. On opening up the body from before there is found to be a tolerably well-developed buccal mass (PI. 20, fig. 12, b. m.) ; the radular sac is of average length, and the salivary glands are somewhat tortuous, simple saccular organs (fig. 12, s.ff.). The radular dentition is strong and of the Littorino-planaxoid type (PI. 20, fig. 8). From a portion of the radula obtained by Smith, Guatkiu referred it to the types approximating to the genus Cerithium, while Smith himself remarked upon its similarity to the radula of the Planaxidae. The oesophagus is long, narrow, and simple, and leads into a large stomachic chamber (PI. 20, figs. 9 and 11, also Fig. I, page 189), on the walls of which there are numerous glandular folds (fig. 11), and a very curious and striking double hemispherical caecum on the floor (fig. 11, and Fig. I, p. 189), between the folds of which the curiously rectangular orifice of a single bile-duct opens (fig. 11, a.h.d.). Besides this stomachic chamber there is an anterior diver- ticulum into which the large stomach opens by a tubular aperture, through which a bristle is represented as passing in fig. 11, w. x., and in this anterior stomachic chamber there lies an almost spherical crystalline style. The intestine passes out of the stomach beneath the tubular aperture between the posterior and anterior stomachic chambers, as indicated by the bristle represented in fig. 11, y. z., also Fig. I, p. 189. The intestine is simple, almost straight, and towards its rectal extremity it contains a number of glandular folds and striae. The liver is large, and occupies the lower two thirds of the last two whorls of the animal's body. There is a single bile- duct, opening, as has already been stated, in the posterior chamber of the stomach. The heart has the normal tsenioglossate characters, and consists of a thin-walled auricle, a thick-walled ventricle, and a short aortic trunk (PI. 21, fig. 1). Between the auricle, ven- THE MOU,USOS OF THE GKEAT AFRICAN LAKES. 191

tricle, and aortic trunks there are the usual valves. The gill of Nassopsis is of average length, very simple in structure, and consists of a large number of low, broad, triangular leaves, the apices of which are not produced into filamentous processes, nor ornamented in any way. The osphradium is long and simple; it lies in a groove at the base of the gill, and shows no tendency to become pectinated or modified in any way either before or behind (PI. 20, fig. 6, o. s.). The nervous system of Nassopsis is extremely interesting, and is certainly one of the most archaic tseiiioglossate types at present known. The cerebral ganglia are widely separated from one another (PI. 20, fig. 7, e.g.); and the pleural ganglia are not only separated from the cerebral ganglia (fig. 10), but quite below the oesophagus, the cerebro-pleural connectives being consequently of great relative length. The supra-intestinal cord springs directly from the right pleural ganglion, passes up over the oesophagus, and carries the supra-intestinal ganglion (fig, 7, sup. int. g.). From the left pleural ganglion there passes a fine nerve towards the supra-intestinal ganglion, which appears to form a dyaloneurous connection with a deriva- tive of the supra-intestinal nerves. Towards the right the Bubintestinal connective passes from the left pleural ganglion beneath the oesophagus straight to the subintestinal ganglion (fig. 7, sub. int. g.). This ganglion is directly connected with the right pleural ganglion by a thick cord (fig. 7, x.), and the nervous system is therefore strongly zygoneurous on the right. Above, the cerebral ganglia give ofF a number of anterior nerves, which are distributed to the buccal mass and the parietes of the head. Among these there are conspicuous the tentacular nerves which pass separately to the tentacles and ocular papillae. The buccal ganglia are situated on the lateral walls of the buccal mass, and are united to the cerebral ganglia by connectives (PI. 20, fig. 7, b.g.). Near the origin of the buccal nerves there arise two fine nerves, one from each cerebral ganglion, which pass forward along the walls of the body, and then bend down, uniting with each other below the mouth (PI. 20, fig. 7, Z«.com.). This connection appears, therefore, VOL. 42, PART 2. NEW SKRIES. N 192 ,T. B. S. MOORE. to be unquestionably the labial commissure described by Bouvier as characteristic of a number of the Archi-tsenioglossate and Rhipidoglossate types. The cerebro-pedal connectives are very long (PI. 20, figs. 7 and 10), and altogether the length of the cerebro-pleural, cerebro-pedal, and pleuro-pedal connectives gives to the nervous system the longi-commissurate character described by Haller. The pedal ganglia are united by a rather small connection, and are prolonged into the foot along the course of two well- developed scalariform pedal cords. Between these pedal cords there exist ladder-like connections similar to those found between the pedal cords of Cyclopherus. The otocysts in Nassopsis are relatively immense (PI. 20, figs. 7 and 10, ot.). They are situated well up on the course of the pedo-pleural connectives, and the otocyst nerves pass obliquely from them towards the cerebral ganglia, and are not quite correctly indicated in figs. 7 and 10. The otoliths are small, numerous, and rectangular, with the faces slightly convex (fig. 10). The reproductive apparatus in Nassopsis is similar in many ways to that of Ty phobia, both male and female apparatus occupying the same general position. In the male the genital gland occupies the upper surface of the apical whorl in the body, and is connected by several channels with a nearly straight vas deferens, represented in PI. 21, fig. 2, v. d. This latter structure opens without any modifications along its course by the slit-like aperture represented in PI. 21, fig. 5. In the female the ovary occupies the same position as the male gland, and in like manner it is connected with the nearly straight oviduct, the lower portion of which, or that which lies within the mantle cavity, forming a brood-chamber where the eggs go through their later stages of development (PI. 20, fig. 6,6. s.).

(B) Bythoceras. Like Nassopsis, the genus Bythoceras, so far as is at present known, is exclusively restricted to Tanganyika, and as a THE MOLLUSCS OP THE GBEAT AFRICAN LAKES. 193

member of the halolimnic fauna of that lake is of considerable interest. Firstly, because, although conchologically so similar to the genera Nassopsis and Paramelania, it has, as we shall immediately see, no morphological relation to the former of these types. By thoceras is interesting, secondly, because it presents us with more numerous points of correspondence with forms such as the genus Tympanotamus, which exists elsewhere, and the anatomy of which is known, than is the case with the majority of the halolimnic forms. By thoceras is at present represented in Tanganyika by a single 6pecies, B. iridescens, which I dredged living at great depths1 in the southern portion of the lake. When young the shell does not possess the characteristic spines represented in PI. 21, fig. 3 (compare PL 3, fig. 4). Nor has it the peculiar pearly thicken- ing of the mouth invariably present in the older forms. In the young condition (PI. 21, fig. 4) the shell is extremely similar to that of Paramelania, and I am inclined to think that the figure of Paramelania crassilabrio given by Professor E. von Martens in his work ' Beschalte Weichtbiere, Deutsch. Ost-Afrikas' (PI. vi, fig. 38), is, in reality, that of a young Bythoceras iridescens. The outward appearance of the animal is extremely similar to that of Cerithium vulgatum, with the exception that there is less pigmentation of the foot, which is nearly white in the Tanganyika species. The snout is short, wrinkled, richly covered with black pigment, and non-protrusible. The tentacles are short, and the eyes are situated on the posterior bases of these organs, and not separated from them on subsidiary papillae as in Nassopsis. The buccal mass is small, and the radular sac short, being reduced, as in the case of Typhobia and Tan- ganyikia, to a small swelling on the floor of the oesophageal tube (PI. 21, figs. 7—11, r.s.)? The radular dentition is extremely interesting, a single row 1 Erom 300 to 1000 feet. 5 Compare figures of Tanganyikia rufofilosa, preceding article (loc. cit.). 194 J. E. S. MOORE. of teeth being represented in PL 21, fig. 9. The "outer and inner lateral teeth distinctly resemble those of the Melano- planaxoid. type which I1 described in considering the relationships of the genus Tanganyikia, and are something similar to those of the " Neomelanian" group of the brothers Saracin ;s while the admedian tooth is peculiar, owing to the presence of two small subsidiary denticles on the inner face (PL 21, fig. 9), a very peculiar feature, and one which is only exemplified in the radula of Tympanotamus. The presence of this peculiarity, together with the general character of the radula, should certainly be regarded as of weight in diagnosing the nearer affinities of this form. And, as we shall see in placing it along with Tympanotamus, it is certainly in accord with the rest of the animal's morphological peculiarities. The oesophagus and salivary glands (PL 21, fig. 7, s. g.) ate in all ways similar to those of Ty phobia, but these characters are common to so many different kinds of Gastropods that they are of little value from a special morphological point of view. The stomach has two chambers, the anterior of which (PL 21, fig. 13) contains a style. The intestine is simple, and takes the course represented in PI. 21, fig. 13. The rectum is not dilated, nor beset with any accessory gland. The bile-ducts seem to open by two very small apertures upon the base of the posterior stomachic chamber. Stomachic valves are feebly if at all developed. The liver is large, and occupies much the same position a.s in Nassopsis (loc. cit.). The excretory organ occupies a place in front of and above the heart, and opens by a minute pore at the extreme upper end of the mantle cavity (PL 21, figs. 5 and 13, r.a.). The heart has the usual tsenioglossate characters, consisting of an auricle, ventricle, and aortic trunk ; but the last structure is much less developed in Bythoceras than in many forms,—as, for example, in the genus Ty phobia.3

1 Loc. cit. * "Die Siisswasser-MoUusken von Celebes " (Wiesbaden, C. W. Kreidel's 'Yerlag'^lSgS. » Loc. cit., ' Quart. Journ. Micr. Sci.,' vol. 41, 1898, p. 190. THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 195 The nervous system in Bythoceras is very interesting, since it is absolutely unlike that possessed by the genus Nassopsis, and closely simulates the type described by Bouvier1 as typical of the genus Cerithium. It also strongly resembles that of the genus Tanganyikia. Viewed from above (PI. 21, fig. 6), the cerebral ganglia are seen to be closely fused together, while the left pleural and subintestinal ganglion, as in Cerithium, form a single massive trunk, which at its hinder extremity gives rise to the subintestinal and visceral nerve-cords, and to the right pallial nerve (fig. 6). From the right pleural ganglion a nerve passes out to the mantle, and a branch from this anastomoses with a branch on the pallial nerve just described. In like manner on the left the pleural ganglion gives birth to a nerve on that side (fig. 6), which passes out and probably anastomoses with a twig given off from the supra-intestinal ganglion, but I was not able to trace this nerve throughout its entire course. Unlike the subintestinal ganglion, the supra-intestinal ganglion is carried on a very long supra-intestinal connective (PI. 21, fig. 6, sup. int. g.) exactly as it is in Cerithium or Aphorais. Viewed from tbe side (PI. 21, fig. 10) the cerebro-pedal and pleuro-pedal connectives are seen to be of considerable length, rather longer than the same structures in Voluta, but not so long as those in Nassopsis or in Strombus. The pedal ganglion has the usual bulbous form as in the true Cerithidse, and in like manner there pass from the lower extremity of each pedal ganglion two predominant foot nerves (fig. 10). The otocysts lie behind the pedal ganglia, and the otocyst nerves pass directly between the cerebro-pedal and pleuro- pedal connectives to the cerebral ganglia on each side (PI. 21, fig. 10). The otocysts are not large, and are round, as dis- tinguished from those of Nassopsis. The otoliths are small, rectangular of barrel-shaped, and numerous. The reproductive apparatus is very simple, and in both sexes consists of a genital gland which occupies the upper 1 'Ann. l)is. Soi. Nat.,' 1887, pp. 131—155, pi. vii, and figs. 196 J. E. S. MOOKE. surface of the last two whorls of the animal's body. This gland is put into connection with a large non-convoluted oviduct or vas deferens, as the case may be, by a number of fine tubes; and both ducts pass beneath the intestine and open just behind the anus in a large slit (PI. 21, figs. 5 and 13,^. a.). The genital duct in both sexes is much enlarged within the mantle cavity, somewhat in the manner of the same structure in the genus Typhobia. But in Bythoceras it is quite destitute of the singular organ which I described as an evertible penis in the male Typhobia.1

Comparative. In considering the phylogenetic relationships of Nassopsis and Bythoceras, it will be needless after the foregoing descrip- tion to insist further upon the fact that these genera bear no relation whatever to each other. Taking Nassopsis first, it will have already been clearly seen that the characters of this singular genus place it unquestionably among the archi-tsenioglossate forms. Whatever opinion one may hold as to the value of the characters of the radula, it will have been seen that they too place it among the more primitive portion of the great melano-planaxoid group ; and beyond this it is doubtful whether the radula can be used in a diagnostic sense at all. The large buccal mass, long radular sac, and the characters of the salivary glands certainly recall the littorinoid group, and at once dissociate Nassopsis from the early Stromboid or Xenophoran type to which Typhobia, Bathunalia, and Tanganyikia all appear to bear more or less distinct affinities. The extremely archaic and simple condition of the whole digestive tract in Nassopsis requires particular attention, for it will have been seen by any one acquainted with my former accounts of the anatomy of Typhohia, Tanganyikia, Bythoceras, and even Spekia, that the digestive apparatus, and especially the stomachic portion of the digestive apparatus in all these molluscs is built upon the same general plan. They all possess

1 ' Quai't. Journ. Micr. Sci.,' vol. 41, 1898, p. 101, loc. cit. THE MOLLUSCS OP THE GREAT AFKIOAN LAKES. 197 anterior stomachic diverticula which contain crystalline styles, similar to those of the genera Strombus, Pteroceras, Ros- tellaria, Mures, and Trochus; and it has already been shown in a former paper1 that there is no reason whatever to doubt that these structures, when they appear in the Proso- branchiata, are like the heart when perforated by the rectum, as in the Trochidse, to be considered as the retention of a condition common to the ancestors of both Prosobranchs and Lamellibranchs alike. The presence of these similar arrangements in so many of the halolimnic Gastropods, which in all other ways are so widely separated from each other, is thus only intelligible if we suppose that peculiar stomachic apparatus was once universal among the ancestors of the Prosobranchiata of the present day, and that in the remnant of an old fauna, such as that now existing in Tanganyika, we encounter a more abundant representation of an archaic state of things.2 The presence of a well-developed mucous gland in Nas- sopsis, the extremely simple character of the gills and the osphradium, the simple reproductive apparatus, are features which all further dissociate Nassopsis not only from the hitherto known fresh-water forms, but also from the more modified Tsenioglossa at present existing in the sea. All the preceding structural peculiarities which demonstrate the archaic character of Nassopsis are, lastly, fully substantiated by the curious condition of the nerves. The great relative length of the cerebro- and pleuro-pedal connectives suggests the condition found in Haller's so-called " longi-commissurate forms/' such as Strombus, Pteroceras, and their allies; while in the presence of a labial commissure we are confronted

1 ' Quart. Journ. Micr. Sci.,' loc. cit., pp. 198, 210. 3 In the case of Nassopsis we can go, however, further than this; for com- paring the spiral caecum in the stomach of this genus with the well-known and similar structure in the stomachs of the Rhipidoglossa, the two are found to be apparently identical, and the retention of this typically primitive feature iu a form which also in ottier ways is so distinctly aichi-teenioglossate is a morphological fact of the highest interest. 198 J. B. S. MOORE. with a feature which, like the caecum, connects Nassopsis on the one hand with the Rhipidoglossa, and on the other with the Archi-paludines. In like manner the wide separation of the cerebral ganglia from one another, and the comparatively great length of the cerebro-pleural connectives, are certainly features which recall the primitive scattered condition of these parts. In fact, the position of the pleural ganglia in Nassopsis represents a condition of development which is really halfway between what I may term the hypo- and the epi-athroid types, the nervous system of Nassopsis marking, in fact, a third, intermediate, or dystenoid type. Again, in the scalariform pedal cords with which Nas- sopsis is provided we have another most important feature, showing that this form is not far from the border-land between the older groups of Taenioglossa and their Rhipidoglossate ancestry. Nassopsis in many ways is more primitive than Palud in a, but at the same time, as will have been seen, it bears no very approximate relation to this form; neither is it very near the ancient group of Littorinas, nor indeed to any of the indi- viduals which at present are regarded as constituting the archi-tsenioglossate group. The morphological interest of Nassopsis lies, therefore, in this, that it presents us with a new starting-point from whence to study the inter-relationships of the great Prosobranchiate order.1 Nassopsis, therefore, presents us with a type of organisation which there are conchological reasons for believing is similar to that possessed by several species of a genus which was once abundant in the sea, but which has long since become extinct outside the confines of Lake Tanganyika, where, 1 I shall elsewhere emphasise more fully the importance of Nassopsis as an archetype, and it is therefore needless for me here to do more than point out the fact that the above conclusions respecting the great morphological antiquity of this form are fully substantiated by the very unexpected similarity which three of the rather marked varieties of the shells of Nassopsis present to the genus Purpurina from the old Jurassic seas. T3E MOLLUSCS OF THE GREAT AFRICAN LAKES. 199 strangely enough, it appears to have lived on, not alone, but in company with several other usually extinct marine Jurassic forms. Turning now to the characters of the new genus Bytho- ceras, we find no such peculiarities as those I have just described as distinctive of Nassopsis nassa. But never- theless this genus is not without much interest from a mor- phologist's point of view. Nowhere, so far as I am aware, have we a better instance of the fact that shell structure as a means of classification is a " delusion and a snare ;" for in the case of Bythoceras it is not only that the shell in no way foreshadows the animal inside it, but that its surprising similarity to Nassopsis is absolutely misleading, and, were we ignorant of the animals contained in both, would lead to a profound error in the close conchological association of the two genera. Who would have dreamed, when contemplating the heavy, thick, arid highly ornamental shell of Bythoceras Howesii, that the animal it contained bore any close resemblance to that enclosed in the strangely different shell of the genus Tympanotamus? Yet in the most peculiar features of its radula, nerves, gills, and viscera it strongly resembles Cerithium, Tympanotamus, and their close allies. The reproductive organs of Bythoceras are simple and peculiar, and in many other ways the structure of this organism will afford ample material for conjecture concerning the final identification of the really primitive cerithoid type. For which is more ancient, Bythoceras or Cerithium? 1 do not feel competent to answer this question, and I would refer the reader to the broader discussion of this subject in my longer paper on "The Prosobranchiate Mollusca" (in the hands of the editor of the 'Quart. Journ. Micr. Sci.'). This much, however, may be affirmed with certainty, that Bythoceras is a form closely related to our general concep- tion of the genus Cerithium, with some of the minor features in its radular dentition peculiar to and distinctive of the genus Tympanotamus. 200 J. JS. S. MOORE. Iii conclusion it may be interesting to reflect how all the evidence which has been collected concerning the nature of the halolimnic Gastropods invariably points to the vast antiquity of these forms. First we have the wide dissimilarity of their empty shells from those of any living types; next their rigid isolation to a solitary great lake, which, judged from whatever staudard we may choose to adopt, is unquestionably of an enormous age. Next we have the wonderful similarity of the halolimnic shells now living in Tanganyika to those which have been left fossilised at the bottom of the old Jurassic seas ; and lastly, there are the morphological characters of the halolimnic animals themselves, whereby they become mentally depicted like nothing so much as the incompletely developed embryos of numerous living oceanic types.

EXPLANATION OF PLATES 20 and 21,

Illustrating Mr. J. E. S. Moore's paper on " The Molluscs of the Great African Lakes."

Reference Letters. a. Anus. g.g. Genital gland, g.a. Genital aperture, m.g. Mucous gland. Int. Intestine, os. Ospliradium. col, m. Columellar muscle, op. Operculum. r.s. Radular sac. s.g. Salivary gland. B. m. Buccal mass. cryst.s. Crystalline st.yle. as. (Esophagus, a.h.d. Aperture of hepatic duct. r. Kidney. 22. Renal aperture, b.s. Brood-sac, g. a. Genital aperture. st. Stomach, ft. Foot, e.g. Cerebral ganglion, pl-g. Pleural ganglion. p.g. Pedal ganglion. I. com. Labial commissure, b.g. Buccal ganglion, ot. Otocyst.. sub. int.g. Subintestinal gangliou. sup. int. g. Supra-intestinal ganglion, os.g. Ospliradial ganglion.

PLATE 20. FIG. 1.—Two views of a variety of the shell of Nassopsis. PIG. 2.—Two views of another variety. FIG. 3.—Embryonic shells and protoconcli of Nassopsis. FIG. i.—Animal of Nassopsis removed from shellj showing the operculum, op. THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 201

FIG. 5.-«-Male Nassopsis, showing the mucous gland, tentacles, and genital aperture. FIG. 6.—Mantle cavity of female Nassopsis, showing character of gill and'osphradium. PIG. 7.—'The nervous system of Nassopsis dissected from above. PIG. 8.—Dissection of Nassopsis, showing relations of stomach, crysta- line style, kidney, (Esophagus, and intestine. FIG. 9.—The radular elements of Nassopsis. FIG. 10.—Nervous system of Nassopsis dissected on the right side. FIG. U.-^Details of the alimentary tract of Nassopsis. FIG. 12,—The relations of the buccal mass, radular sac, and salivary glauds of Nassopsis.

PLATE 21. FIG. 1.—The relation of the heart, pericardial cavity, and reno-pericardial connection in Nassopsis. FIG. 2.—The reproductive apparatus of a male Nassopsis. FIG. 3.—Two views of the shell of Bytboceras Howesii. FIG. 4.—Young shell of Bythoceras Howesii, showing the absence of the spines above and below the mouUi. FIG. 5.—Dissection, showing the gill and mantle organs of Bythoceras. FIG. 6.—The nervous system of Bythoceras dissected from above. FIG. 7.—The buccal mass, radular sac, and salivary glands of Bythoceras. FIG. 8.—The nervous system of Cerithium vulgatum dissected from above. FIG. 9.—The radular elements of Bythoceras. FIG. 10.—The nervous system of Bythoceras dissected from the right side. FIG. 11.—Bight side view of the buccal mass, salivary glauds, and radular sac of By thoceras. FIG. 12.—The operculum of Bythoceras. FIG. 13.—Details of the aliineutary apparatus of Bythoceras. If.CO.

Col.rrv. x 3*