mantle in the common cockle, which is regularly raised and depressed at short intervals, corresponding to the grooves and ribs of its shell. But very often these fleshy appendages of the mantle appear to be developed only at certain periods, and, after the orgasm is over, again to fall and subside into inactivity. In their times of development cases of calcareous matter are formed for their protection, and, before the animal proceeds to add a new piece to its domicile, it fills up these cases more or less completely with shelly matter, which then remain, in the form of crossridges, to constitute the shell's principal feature and beauty. In this manner are formed the vaulted spines on many Cockles; the strap-like processes of the Spondyle; the thick ribs that gird many Rock-shells; the spinous, pectinated and foliated processes of many other Muricidæ ; and, in short, all the irregularities and inequalities which appear, at interrupted intervals, on the surface of every shell.

But although, from the preceding remarks, you might expect to find an index to the animal's organization in the exterior aspect and model of the shell, yet you must consult the index with heedful caution, for if solely depended upon, it will occasionally mislead you. This is now admitted. Montagu affirms, "that similar shells are sometimes inhabited by very different animals.” * Cuvier has frequently insisted on this fact; and Mr. Collier goes so far as to say, that "many Mollusca, alike in form and structure, inhabit shells so essentially different in character, as to render the union of the two modes of distinction impossible." In a book which happens to lie on my table, I find an apposite example in a new species of Mollusk, found by Mr. Alder at Dalkey Island, near Dublin. The shell was in all respects similar to that of the genus Rissoa, but the snail had four, and not two, tentacula; and the eyes were placed on the back at some distance behind the tentacula, and not at their base. This subject, which is one of considerable importance to the geologist, has been handled so ably by Mr. J. E. Gray, that I cheerfully avail myself of his permission to send you his remarks, unmutilated.

*Test. Brit. pref. iii.

+ Edinb. New Phil. Journ. vii, 225. Reports Brit. Assoc. 1843, Trans. p. 74.

DISSIMILAR ANIMALS IN SIMILAR SHELLS.

403

"Of Shells apparently similar, but belonging, on a comparison of their Animals, to very different Genera." By JOHN EDWARD GRAY, Esq., F.R.S., &c. (Reprinted from the Philosophical Transactions, part ii. 1835.)

"In a note on my former paper on the structure of shells, I pointed out the perplexity in which the extreme similarity of the shells belonging to the genera Patella and Lottia must involve the geologist and the conchologist, intending at some future time to pursue the subject further, and to show that similar difficulties existed in regard to several other genera. The two genera above referred to are probably, however, the most remarkable example of this complete resemblance, on account of the extreme dissimilarity of their animals, which are referable to two very different orders of Mollusca, while the shells are so perfectly alike, that after a long-continued study of numerous species of each genus, I cannot find any character by which they can be distinguished with any degree of certainty. Both genera present a striking discrepancy from all other univalve shells, in having the apex of the shell turned towards the head of the animal, the genera to which they are immediately related in both the orders to which they belong offering no variation in this respect from the usual structure of the class. The agreement in the internal structure of their shells is equally complete; yet the animal of Patella has the branchiæ in the form of a series of small plates disposed in a circle round the inner edge of the mantle, while that of Lottia has a triangular pectinated gill seated in a proper cavity formed over the back of the neck within the mantle, agreeing in this respect with the inhabitants of the Trochi, Monodontæ, and Turbines, from which it differs so remarkably in the simple conical form of its shell. This difference in the respiratory organs of animals inhabiting shells so strikingly similar is the more anomalous, inasmuch as those organs commonly exercise great influence on the general form of shells; a circumstance readily accounted for when we reflect that a principal object of the shell is to afford protection to those delicate and highly important parts.

"To the practical conchologist it will be sufficient to mention Pupa and Vertigo, Vitrina and Nanina, Rissoa and Truncatella, as affording numerous and perplexing instances of the difficulty of distinguishing between genera of shells, inhabited by very different animals.

* Philosophical Trans. 1834, p. 800.

Iridinæ and Anodontæ differ in the adhesion and non-adhesion of the lobes of the mantles, yet the shells are so alike that they cannot be distinguished by any external character; so much So, that one of the species now referred to the genus by M. Deshayes, who first pointed out this peculiarity in the animal, was considered as an Anodon by Lamarck.

"The animals of Cytherea, Venus, and Venerupis have, like those of most of the allied genera, a lanceolate foot projecting at the anterior part of the shell; while the genus Artemis of Poli, which has generally been confounded with Cytherea, from which it is not easily to be distinguished except by its usually more rounded form, is provided with a crescent-shaped foot, exserted at the middle of the lower edges of the valves.

66

'Again, there is but little difference in external characters and habit between Cyclas and Pisidium; but the animals of the latter have elongated siphons which are not found in the former.

"In reference to Univalves it may also be observed, that it is frequently impossible to distinguish some of the genera of that class without an examination of their opercula. This is the case, for instance, as regards the smaller and more solid Paludinæ, inhabitants of fresh water, and some species of Littorina living on the coast; several of the shells described as Paludina by Drapernauld and others appearing rather to belong to the latter genus. A similar difficulty exists with respect to other Littorinæ as distinguished from Phasianella, and with the Neritinæ as distinguished from the Neritæ. In the latter case the characters derived from the operculum are so essential to the discrimination of the two genera, that M. Rang, looking only to the characters of the shell, has proposed to reunite them into one. In proof of the little attention that has hitherto been paid to this very important part, I may mention that three species referred by Lamarck to the genus Solarium are each furnished with a different kind of operculum; and it is deserving of notice that the Monodonta canaliculata, according to the observations of M. Quoy, has an operculum very different from the rest of the shells of that genus.

"In some shells, again, the differences in character are so slight as almost to throw an air of ridicule on the attempt to separate them generically from the structure of the shells in most of the specimens of the species which he describes. The English conchologists, misled by this character, have referred to the genus a very different African shell, with a long series of transverse teeth on the hingemargin, which has lately been separated by Mr. Conrad under the name of Pleiodon.

alone; and yet when the animal is examined the necessity of their separation becomes so obvious as to be immediately acknowledged. This is especially the case with my genus Bullia compared with Terebra: the shells of these two genera are so similar, that Lamarck and all other conchologists have retained them in one group, no other distinction being observable except that in the former there is a more or less distinct callous band winding round the volutions just above the suture, and produced by a slight extension of the inner lip beyond the part of the shell occupied by the whorl. This extension of the lip is probably deposited by the foot of the animal, which in the genus Bullia is very large and expanded, while that of Terebra is small and compressed. This, however, is not the only difference between the two animals, that of the former genus having rather large and eyeless tentacles, while the Terebræ have very small and short tentacles, bearing the eyes near their tips.

"A second example of a similar kind is derived from the genus Rostellaria, in which Lamarck includes the Strombus pes-pelecani of Linnæus. The animal of this shell has been figured by Müller, and very much resembles that of Buccinum, having long slender tentacles with the eyes sessile on the outer side of their base; while, as Dr. Rüppell informs me, the Rostellaria curvirostris has an animal allied to Strombus, with the eyes on very large peduncles, which give off from the middle of one of their sides the small tentacles. Notwithstanding this difference in the form of their animals, I am not, however, aware of any essential character by which the shell of Aporrhais (as the Strombus pes-pelecani has been generically named) can be distinguished from the other Rostellariæ.

"With all this uncertainty with regard to the generic characters of the recent species of shells, of which the animals can be subjected to examination, how much must the difficulty of deciding their genera with certainty be enhanced with reference to the fossil species, and especially to those which have no strictly analogous form existing in the recent state. Considerations like these tend greatly to disturb the confidence formerly reposed in the opinion that every difference in the form and structure of the animal was accompanied by marks permanently traced upon the shell, by which it might be at once distinguished, and which it was therefore the great object of the conchologist to point. out."

408

LETTER XXIII.

ON THE FORMATION AND STRUCTURE OF SHELLS.

BY JOHN EDWARD GRAY, ESQ., F.R.S., &c.

** Reprinted by permission of the Author from the Philosophical Transactions for 1833.

1. First Formation of Shells.

THE shells of Mollusca appear to be coeval with the first formation of the animal: they may be observed covering the embryo on its first development in the egg, even before it has acquired its proper shape or any of its internal organs. The accurate Swammerdam observed them in the eggs of several of the garden and pond snails. His observations have been recently verified and extended by Pfeiffer, on many species of land and fresh-water Mollusca; and I have myself observed the same fact in the eggs of several animals belonging to the different orders of marine shells; there is reason, therefore, to believe that this circumstance is general throughout the class. These observations are most easily made on the embryo of the fresh-water shells, such as the Lymnææ, Physæ, Ancyli, and Bithyniæ, the eggs of these animals being covered with a transparent coat; while the viviparous Mollusca, and especially the Littorinæ, Paludinæ, and Cyclades,* offer the additional advantage of exhibiting the embryos of their animals in all the different states of development at the same time.

*Between the lamina of the branchiæ of the Anodontes and Uniones are found small cordate, bivalve bodies, which have been considered as their young; but they differ so much in external form and internal structure from the adults, that many excellent naturalists, and especially Professor Jacobson, of Copenhagen, have considered them as parasites. It is, however, remarkable that they are found in abundance in almost every specimen, and Pfeiffer has apparently proved that they are the young, he having found them constituting the umbones of very minute Uniones. I have searched for them in vain in this situation; perhaps because I have never been so fortunate as to discover specimens of the young shell so small as those figured by this author. If Pfeiffer should prove to be correct, this remarkable change of form and structure will be the only approach towards a metamorphosis that has been hitherto observed in this class of animals.