The shells of Mollusca may always be regarded as epidermic in their cha racter ; being formed upon the surface of the mantle, which answers to the true skin of other animals. As might be anticipated from this description, they are essentially composed of cells, consolidated by a deposit of carbonate of lime in their interior ; but, as in other tissues, we frequently find that the original cellular organisation is obscured by subsequent changes, and we sometimes lose all traces of it. We shall first examine it in what may be consi dered its typical condition, which is most cha racteristically seen in certain bivalves.
If a small portion be broken away from the thin margin of the shell of any species of Pinna, (this margin being composed of the outer layer only, which projects beyond the inner), and it be placed without any prepa ration under a low magnifying power, it pre sents on each of its surfaces, when viewed by transmitted light, very niuch the appearance of a honeycomb ; whilst at the broken edge it exhibits an aspect which is evidently fibroa to the eye, but which,when examined under the miscroscope with reflected light, resembles that of an assemblage of basaltic columns. The shell is thus seen to be composed of a vast number of prisms, having a tolerably uniform size, and usually presenting an approach to the hexagonal shape. These are arranged perpendicularly (or nearly so) to the surface of the lamina of the shell ; so that its thick ness is formed by their length, and its two surfaces by their extremities. A more satis factory view of these prisms is obtained by grinding down a lamina until it possesses a high degree of transparency ; and it is then seen (fig. 407.) that the prisms themselves appear to be composed of a very homogeneous substance, but that they are separated by definite and strongly-marked lines of division. When such a lamina is submitted to the action of dilute acid, so as to dissolve away the carbonate oflinie, a tolerably firm and consistent membrane is left, which exhibits the prismatic structure just as perfectly as did the original shell (fig. 408.) ; the hexagonal divisions being evidently the walls of cells resembling those of the pith or bark of a plant, in which the cells are fre quently hexagonal prisms. In very thin natural laminm, the nuclei of the cells can often be plainly distinguished ; but we cannot expect to find these, when the two ends of the cells (at one of which they are generally situated) have been removed by grinding. By making a section of the shell perpendicularly to its surface, we obtain a view of the prisms cut in the direction of their length (fig. 409.) ; and it is then seen that whilst many of them pass shells, that the decay of the animal membrane leaves the contained prisms without any con continuously from one surface of the layer to the other, some terminate in points midway, necting medium ; and being then quite lated, they can be easily detached from one Hence it happens that the nunzber of the re ticulations is smaller on the interior than on the exterior of the layer ; their size, on the contrary, being greater. The 'prisms are seen
to be marked by delicate transverse stri, closely resembling those observable on the prisms of the enamel of teeth, to which this kind of shel!-structure may be considered as bearing a very close resemblance, except as regards the mineralising ingredient. If a si milar section be decalcified by dilute acid, the membranous residuum will exhibit the walls of the prismatic cells viewed longitudi nally ; and these will be seen to be more or less regularly marked by the transverse stri just alluded to. It sometimes happens in recent, but still more commonly in fossil another without any fracture. A group of three such prisms, found in a fragment of chalk, is shown in fig. 411.: it is seen that these also exhibit transverse strke of a si milar aspect. By submitting the edges of the membranous walls of the prismatic cells divided longitudinally (as infig. 410.) to a high magnifying power, the cause of the transverse thickness of the shell is made up of the nal or nacreous layer ; but a uniform stratum striation is seen to be a thickening of the wall in those situations ; which will of course produce a corresponding series of indentations upon the contained prisms. This thickening seems best accounted for by supposing (as first suggested by Prof. Owen) that each long prismatic cell is made up by the coalescence of a -pile of flat epidermic cells, the transverse striation marking their lines ofjunction ; and this view corresponds well with the fact that the shell-membrane not unfrequently shovvs a tendency to split into thin laminm along the lines of striation, as shown in the lower part offig.410; whilst we occasionally meet with an excessively thin natural lamina, composed of flat pavement-like cells resembling those of the epithelium of serous membrane, lying between the thicker prismatic layers, with one of which it would have probably coalesced but for some accidental cause which pre served its distinctness. That the entire length of the prism is not formed at once, but that it is progressively lengthened and consolidated at its lower extremity, would appear also from the fact that where the shell presents a deep colour (as in Pinna nigrina) this colour is usually disposed in distinct strata, the outer portion of each layer being the part most deeply tinged, whilst the inner extremities of the prisms are almost colourless.