Dr. Carpenter describes the appearances here figured in the following terms (/. c. p. 8.): " Although the prismatic cellular structure has not yet been actually observed in process of fortnation, yet certain appearances, which are occasionally met with in the marginal portions of its newest layers throw great light upon its mode of growth', and indicate its strong resemblance to cartilage in this respect ; for in these situations we find the cells neither in contact with each other, nor polygonal in form, but separated by a greater or less amount of intercellular substance, and presenting a rounded, instead of an angular form (fig.314. c). Upon looking still nearer the margin, the cells are seen to be yet smaller and more separated by intercellular substance, and not unfrequently we lose all trace of distinct cells, the intercellular sub stance presenting itself alone, but containing cytoblasts scattered through it. This appear ance has been noticed by myself in Pinna and Unio, and by Mr. Bowerbank in Ostrea; so that I have no doubt that it is general in this situation. We may, I think, conclude from it that the cells of the prismatic cellular sub stance are developed, like those of cartilage, in the midst of an intercellular substance, which at first separates thetn from each other, that as they grow and draw into themselves the carbonate of lime poured out from the subjacent surface, they approach each other more and more nearly ; and that, as they attain their full development, their sides press against each other, so that the cells acquire a polygonal form, and the intercellular substance disappears." I have given Dr. Carpenter's statement at length, because it appears to me to express very distinctly the interpretation which one is at once tempted to put upon the appearance, but which I must reject for the following reasons :—In the first place, if we examine that portion (a) of the margin beyond the smallest granules (aytoblasts, Carpenter), it is seen to be either absolutely structureless or obscurely striated, not a trace of a cell or endoplast being anywhere visible. Secondly, if any dilute acid be added under the micro scope, the apparent nuclei and cells vanish with effervescence, and leave behind them clear empty spaces, of exactly the same shape and size as they themselves had. Thirdly, the supposed cells have a peculiar concen trically or radially-striated structure, resem bling sections of urinary calculi on a small scale, and still more the corresponding bodies in the integument of the shrimp (supra.) For these reasons I think it must be granted that the appearances in question, hovv ever cell-like, are, in reality, not the expres sion of the development of a cellular struc ture at all, but merely that of the mode in which the deposit of calcareous matter takes place in the membranous basis of the shell. In fact, I believe that the calcareous matter appears first in small and distinct globules (the " cytoblasts"), and that more or less concentric deposits take place round these, the result of which is, that the membranous basis is more and more displaced, and that the deposited masses eventually come almost into contact. The regularity of the ultimate pris matic structure results from that of the dis tances of the granules primarily deposited, and the even rate of addition to each subse quently.
There appears to me to be but one inter pretation to be placed upon these facts; viz. that cells as such do not enter into the for mation of the shell of the Naiades at all, but that it is constituted by the successive excre tion of ntetnbranous lantinw from the surface of the epidermis of the mantle.* The outer !maim. retain their membranous nature, only becoming so far altered as to assume the horny aspect of the so called " epidermis ; " in the next laminm, which are added to the inner surface of the young shell, calcareous matter is deposited in granules, additions to which are made in such a manner as to con stitute the cellmform concretions, and ulti mately, the process going on in the same way in successive layers, the prisms; in the inner most laminm, finally, the calcareous deposit results in an even, homogeneous, folded or striated layer. By scraping with a sharp knife the inner surface of the shell of Anodon, freshly detached from the mantle, I have obtained a distinct tough membranous layer, scattered through which were a vast number of close-set irregular granules of calcareous matter. A similar structureless layer without the granules constitutes the outermost surface of the ecderon of the mantle (fig. 313. c, 6') and may occasionally be detached as such.
Such a layer consisting of the thickened outer portion of the periplast of the ecderon of the mantle is by no means an anomalous structure, as we have a formation of exactly the same kind in the " cuticle " of plants, and in the chitinous lining of the intestine in Insects ; and I believe that the shells of mollusks in general consist simply of a multitude of thin layers successively thrown off, super-imposed and coherent, all the peculiarities of their struc ture arising from subsequent modifications, which are altogether independent of cells. This view is in perfect agreement with all that is known of the nature of the shells of larval Gasteropods and Acephala, which are invariably either of an absolutely structureless, thin, transparent, membranous character, or at most present a delicate striation. It may be added that not the slightest trace of a cel lular structure is to be met with in the pellucid shells of the Heteropoda and Pteropoda. So ntuch for the two primary forms of shell struc ture, the membranous and the prismatic. A most interesting variety of the former is the nacreous (mother-of-pearl) lining which is presented by many shells, both of Ace phala and Cephalophora. The pearly iri descence proceeds, as Dr. Carpenter has well shown, from the folding of the membranous layer into close plaits, and not, as has been supposed, from the alternate cropping out of calcareous and membranous layers. Dr. Car penter proved this by decalcifying with acid a layer of nacre from Haliotis splendens. The iridescence remained; but if the plaits of the layer were pulled out by stretching it with needles, the iridescence disappeared.
Another variety of structure usually, but not alone found in the membranous shell substance, is the tubular. "All the different forms of membranous shell structure are oc casionally traversed by tubes which seem to commence from the inner surface of the shell, and to be distributed to its several layers. These tubes vary in size frotn about the 20.00 to the TA, of an inch, but their general diameter in the shells in which they most abound is about of an inch. The di rection and distribution of these tubes are extremely various in different shells; in general, when they exist in considerable numbers, they form a network which spreads itself out in each layer nearly parallel to its surface, so that a large part of it comes into focus at the same time in a section which passes in the plane of the lamina. From this network some branches proceed towards the nearer side of the section as if to join the network of another layer, whilst others dip downwards, as if for a similar purpose " (Car penter, /. c. p. 14.). In other instances the tubes run obliquely through all the layers. The forraer structure was found by Dr. Carpenter in the outer yellow layer of Anomia plum ; the outer layer of Lima scabra and in C'hanta, the latter in Arca, Pectunculus, and Trigania. In the latter case, the tubules are not continuous, but are seen under a high power to be formed by rows of isolated vacuities, one for each lamina; corresponding, I imagine, with the appearance, " as if they had arisen from the coalescence of lineally arranged cells," pointed out by Mr. Bowerbank and Dr. Carpenter. Having already given what are, I believe, sufficient reasons for denying the existence of' cells of any kind in molluscan shells, I need hardly add that I cannot think this to be the true explanation of the mode of development of these tubules. In fact, I con sider that the tubular shell structure is iden tical with that of dentine, and has precisely the same origin ; its tubuli arising not from cells, but like the canaliculi of bone, by a process of vacuolation in the calcified tissue. I regard the stnicture and mode ofdevelopment of the Molluscan like that of the Annulose shell, in fact, as evidence of the strongest and rnost unmistakable kind in favour of the views with regard to the formation of dentine which I ventured to put forth in ray essay " On the Development of the Teeth." Tooth and shell cotnpletely represent one another, structure for structure; Nasrnyth's membrane is the homologue of the " epidermis," the enamel that of the prismatic structure, the dentine, that of the membranous structure ; and all three are produced without the inter vention of cells by the differentiation of pri marily structureless laminm. The existence of tubuli in the prismatic substance is not mentioned by Dr. Carpenter, but I have no ticed them very distinctly in one of the sec tions of Pinna from his cabinet.