is difficult to imagine, therefore, how any communication could have existed between the animal contained within the shell, and the cancellated structure which forms its thickness." Now, as to the formation of these structures, Dr. Carpenter is at antipodes with Mr. Bowerbank in opinion. He views the shell as being not analogous to bone, but as the representation in the Mollusca of the cutaneous membranes, the view which was taken of their relation to the animal by Cuvier and his followers. We may pass over, in our statement of the theory, the formation of the basement or primary membranes by the successive production and coalescence of cytoblasts and cells, for this is merely a part of the general theory of the formation of membranes, but commence with the process after it has proceeded beyond this initiative. I gather, then, from Dr. Carpenter's essays, that he con+ siders the periostracum or skin of the shell to be cast off from the animal as an epidermis, and to be, consequently, inorganic. Beneath it an organized membrane or epithelium, derived from the mantle, is then detached, and, by the secretion of calcareous matter in its cells, is hardened into a layer of shell, which layer is thus nothing else but a calcified epithelium analogous with the enamel of the teeth. By the rapid and successive production of similar layers of exuvial membrane and of lime within its cells, the shell is ultimately completed in the following manner. The shell of the animal in the egg consists, perhaps, only of periostracum, and at most merely of an additional layer of epithelium in which no lime has as yet been deposited. By the growth of the animal a new edge of periostracum is added to the rim of that already formed, and underneath it a new rim also of calciferous epithelium, enlarging as a matter of necessity the shell in the direction of the animal's growth, and with an equal step. The thickening of the shell is at the same time carried on in the parts already traced by the deposition of calcareous layers,—a deposition which is the result of the casts of the secreting epithelium furnished by the underlying mucous skin of the mantle. The periostracum and cellular layers are then made by an extension of the margins of the aperture or valves by a secretion from the collar or edges of the mantle; but the nacreous layers proceed from secretions furnished by the mantle itself. * Mem. xi. 8. "Ces deux valves primitives sont d'abord purement membraneuses. Le compresseur les aplatit sans les rompre," &c.-A. DE QUATREFAGES, Embryogénie des Turets in Ann. des Sc. Nat. (1849) xi. 211. I may advantageously lengthen this explanation by employing more exactly the language of Dr. Carpenter. He tells us that all shells are generated in the first instance by the agency of the epithelium of the mantle, the cells of which have the power of consolidating themselves by drawing calcareous matter into their interior. By successive casts of this exuvial membrane, the laminæ of the shell are increased in numbers. "The margin only," says Dr. Carpenter, "of the mantle, has the power of giving origin to the outer layer of the shell, whilst its whole surface may generate the inner. Every new production of shell consists of an entire lamina of the latter substance, which lines the whole interior of the old valve, and of a border or margin of the former which thickens its edge. So long as the animal continues to increase in dimensions, each new interior layer of shell projects so far beyond the preceding that the new border, composed of the outer layer, is simply joined on to the margin of the former one; so that the successive formations of the outer layer scarcely underlie each other. But when the animal has arrived at its full growth, the new laminæ cease to project beyond the old; and as each is still composed of a marginal band of the external substance, attached to the edge of an entire lamina of the inner, these bands must now underlie each other, being either quite free as in Ostreæ, or closely united to each other as in Unio and most other bivalves."-The additions to the shells of the Gasteropods are made upon the same plan. Admitting the justness of this explanation it follows "that no addition can be made to the outer stratum after the subjacent layer has been formed, except by a deposition upon its external surface, as in Cypræa: nor can any change be made in the thickness of the middle stratum after the formation of the internal layer. But this last-formed internal layer may be thickened by successive deposits to any extent; and this appears to me to be the explanation of the fact that the thickness of the internal layer at some distance backwards from the lip, bears a considerably greater proportion to that of the middle and external layers than it does nearer the margin. "Upon measuring," says Mr. Bowerbank, "the relative degrees of thickness of the strata at different parts of the same shell, I found the following variations. The thickest part of the stratum of plates near the mouth of a young Ampullaria, which were parallel to the lines of growth, may be represented by the number 25. At half the revolution of a whorl backwards the same stratum was represented by 20, while the new stratum beneath it measured 13; total 33. At a whole revolution backward the upper stratum measured 18, and the lower or new one 20; the older of the two thus appearing to decrease gradually in thickness as the younger grew in substance. Upon examining, in a like manner, an adult specimen of Bulimus oblongus, the proportions of the strata near the lip were as follows:-Outer 10, central 14, inner 17; total 41. But at one revolution backwards they were-outer 5, central 5, inner 19; total 29." This fact harmonizes well with the preceding explanation, for it simply indicates that whilst the thickness of the whole shell undergoes a considerable increase near the lip, that increase is due to the greater developement of the outer and middle layers, the inner layer being positively thinner than it is at a distance from the lip, where it has been thickened by successive deposits. I can see no reason to attribute the foregoing differences in the relative thickness of the three layers at different parts of the shell to any absorption or removal of the outer layers, as Mr. Bowerbank seems inclined to do, for it seems impossible to imagine that the external and middle layers can be thinned by absorption without the removal of the inner layer, since any absorbent action must take place from within, being effected by the surface of the mantle. I am far from denying that such absorption does take place; but the explanation is not applicable to the facts just cited, which seem to me to indicate clearly that the formation of the inner layer is progressive, and not completed at one effort. The truth appears to me to be, that whenever an addition is made to the shell, the outer and middle layers are simply joined on to the edges of the old margin, but that the internal layer is carried backwards for a certain distance into the interior of the shell, where the new formation forms a lining to the old, and increases its thickness just as in the bivalves. I have never been able, however, to trace it very far back, and it certainly can seldom or never line the whole shell, as it does in most bivalves. But this new layer seems to cover that part of the internal surface which is in contact with the moving parts of the animal; and thus serves to prevent that irregularity which could scarcely fail to exist were the new internal layer, like the middle and external, simply joined on to the edge of the preceding."* Dr. Carpenter's Essays on the Microscopic Structure of Shells are contained in the Reports of the British Association for the Advancement of Science, 1843, p. 71; 1844, p. 1-23; 1847, p. 93-117; and in Ann. and Mag. N. Hist. xii. 377---86. The theory, as thus expounded by Dr. Carpenter seems to be the true one, and it recommends itself the more that it can be easily reconciled with the experiments of Reaumur. Indeed, the theory of the French naturalist differs in little or nothing from the one now proposed except in considering the excreted lime as consolidating itself from a mechanical cause, instead of being moulded in the cells of a secreting membrane. Hence there is much in the old doctrine which needs no change of expression to suit it to the newer physiology; and this in particular is the case with that part of it which relates to the colouring of the shell. The colour is situated always in the outer layers, partially dimmed in many by the periostracum, and, therefore, it must be furnished solely from miliary glands situated in the collar or edges of the mantle. All the varieties of colours, and all their varied and mixed patterns which render shells so attractive and pleasing, are the result of the arrangement of these glands, and on their secretion of the colouring matter being uninterrupted, or interrupted at regular intervals. In a great number of Mollusca it would appear that the increase of the shell from birth to mature size is uninterruptedly progressive; but there are, perhaps, an equal number, in which the animal, at certain and determined intervals, forms a transverse rib or varix, and seems to become for a season inoperative. These ribs vary much in their numbers, in their figure, and in the distances at which they are placed in different shells, but in the same species are alike and uniform, so that it would perhaps be better at once to refer their formation to a law of their individual life imprinted on them by their Creator, than to seek for its explanation in causes which are only partially applicable, or of doubtful existence. We may suppose, indeed, with De Montfort and Blainville, that, during the season of love, the derivation of fluid and of energy to the generative system may diminish the secretion of fluid and of lime from the cloak, and that then the growth of the shell goes on as usual, as is indicated by the plain intermediate spaces. We may further suppose, that, when the seminal turgescence has subsided, the fluids are carried in greater abundance to the skin, whence an accumulation of calcareous matter in the margins of the collar, and a consequent varix or rib. We may make such or similar suppositions, but they are idle and improbable; and I men * Lamarck supposes that on the addition of every new piece which the growth of the animal obliges it to make to its shell, the Ranella comes out and exposes itself for the entire length of an half volution, and thus remains tion the hypothesis rather from respect to its able advocates than from any the slightest conviction of its truth. It is inconsistent with analogy to believe that the Mollusca are influenced by the sexual passion long previous to the attainment of maturity; yet the hypothesis assumes that some of them feel its power almost from the date of their birth, and afterwards at very short and frequent intervals; while others, of the same genus even, are swayed by it at distant periods, and only two or three times during the term of their existence. And in what predicament are those which are plain and ribless? Are we to believe that their life is love unceasing; or that it begins only when the animal reaches maturity, and is about to finish the aperture of its edifice? In the progress of its formation, the shell of every class is moulded on the mantle of the animal, and when placed in your cabinet you have in the shell a permanent cast of the form and the main peculiarities of the latter. Every line and plait of the mantle is most accurately imprinted on the inner surface of the shell, so that from its examination we may often derive very correct information relative to the inhabitant's organization, and some of the most assured characters for distinguishing the genera. You may say to the shell by and bye, when your knowledge is riper,— "There is a kind of character in thy life That to the observer doth thy history Whenever the edge of the mantle is furnished with any fold or protuberance, with processes or beards, corresponding processes on the shell declare the fact; and these processes are cast in the form of cases for the protection of the fleshy parts they represent. If the original make of the mantle continues during life invariably alike, the surface of the shell is conformable and uniform, either smooth and even, or marked with striæ and ridges that extend from the apex to the edge in uninterrupted lines. You may easily satisfy yourself of this by examining the collar of the common snail, which is as even as its shell, and the edge of the stationary until the new half volution is formed; a fact, he says, which is proved by an examination of the shell, and evidenced by the varices being constantly disposed on the two opposite sides. I have heard this opinion maintained by conchologists, but it cannot be even discussed by those who have studied the theory of the formation of shells. Sowerby and Deshayes have animadverted on the hypothesis of Lamarck sufficiently. Gen, of Rec, and Fossil Shells in verb. Ranella: Lam. Anim. s. Vert. 2nde edit. ix. 538. D D |