The prismatic arrangement of the carbonate of lime in the shells of Pinna and its allies has been long familiar to conchologists; but it has been usually regarded as the result of crystallisation. It is now, however, perfectly evident that the calcareous prisms are nothing else than casts of the interior of the prismatic cells ; the form of which, however irregular, they constantly present ; whilst the markings of the membrane are faithfully transferred to the surface of the prism. Further, the prisms in a thick layer of shell frequently present a decided curvature, which would not be the case if their form were due to crystallisa tion. Not unfrequently, moreover, they are altogether destitute of angular boundaries ; the large quantity of animal matter disposed between the contiguous cells giving them a rounded contour, as seen infig. 412, and thus causing the calcareous casts of their interior to be cylindrical rather than prismatic.
It is only in a few families of Bivalves, how ever, that the cellular structure is seen in this very distinct form, or that it makes up a large part of the substance of the shell ; and these families are for the most part nearly allied to Pinna. In all the genera of the Margaritacece, we find the external layer of the shell formed upon this plan, and of considerable thickness ; the internal layer being nacreous. In the Unionidce, on the contrary, nearly the whole of prismatic cellular substance is always found between the nacre and the periostracum. In the Ostracece the greater part of the shell is composed of a sub-nacreous substance, the successively-formed laminm of which have very little adhesion to each other ; but every one of these laminm is bordered at its free edge by a layer of the prismatic cellular substance, distinguished by its brownish-yellow colour : this structure presents itself again in the family Pandoridce, which belongs to quite a different section of the class ; and it is curious to ob serve that the marked difference in the struc ture of the shells of Pandora and Lyonsia from that of the Anatinidce and other neighbouring families, harmonises completely with the pecu liar combination of characters presented by the animals of these two genera.* In all the foregoing cases, a distinct cellulo-membranous residuum is left after the decalcification of the prismatic substance by dilute acid ; and this is most tenacious and substantial where, as in the Margaritacece, there is no proper perios tractim,— as if the horny matter which would have otherwise gone to form this investment had been diffused as an intercellular substance between the proper cell-walls.
In many other instances, a cellular arrange ment is perfectly evident in sections of the shell ; and yet no corresponding structure can be distinctly seen in the delicate membrane left after decalcification. In all such cases, the animal basis bears but a very small propor tion to the calcareous deposit, and the shell is usually extremely hard. A very characteristic example of this is presented by the outer layers of the shells of the genus Thracia and other Awitini&r. But there are numerous other cases, in which no traces of cellular structure can be detected in the fully-formed shell, and in which we can only be guided by analogy in assigning to them a similar origin with the preceding. We seem justified in doing so, however, by two considerations. In the first place, where the fully-formed shell is destitute of cellular arrangement, this may be frequently detected in the embryonic shell ; as the author is informed by Dr. Leidy of Philadelphia, who has carefully studied the embryology of many Mollusca. And secondly, there are certain shells which exhibit so complete and gradual a transition from adistinct cellular arrangement to an apparently homogenous structure, that we can scarcely doubt the common origin of both substances. This is particularly well seen in the common 'Ilya arenaria, a careful examination of which shell brings to light nu merous interesting varieties of cellular organ isation. Thus in ,fig. 413. we see in one part of thus it is found occasionally in Anomitt and Peden, and generally in Chama, Cleidothwrus, the section a very distinct set of cell-boun daries, with a large nuclear. spot in the centre of each cell ; whilst on the other side we ob serve that the cell-walls have completely dis appeared,—the nuclear spots, however, still remaining to mark the cellular origin of the substance. A little further on, these also might disappear, and thus all traces of the original organisation might be lost, though no reasonable doubt could be entertained as to its prior existence. A very curious variety of cell-structure is seen in the large hinge-tooth of Mya, in which there is a layer of large cells occupied by carbonate of lime disposed in a radiated form of crystallisation, resembling that of the mineral called Wavellite. Ap proaches to this beautiful arrangement may be seen in many other shells. Here, too, we find the partitions between the cells gradually becoming less distinct, as we pass from this peculiar stratum into the surrounding substance, until we lose them altogether. In general, a cel lular layer may be detected upon the external surface of bivalve shells, where this has been pro tected by a periostracurn, or has been prevented in any other mode from undergoing abrasion : Trigonia, Anatina, Solen, Glyeimeris, Solemya, &c. In the last-named genus it is very firm,
and leaves a distinct membranous residuum after the calcareous matter has been removed by acid, which is not the case with the others. The cells of which the outer layer of the shell is made up are frequently ratherfusiform than prismatic in their shape, and are disposed with their long axes nearly parallel to its surface, so that their extremities " crop out" very ob liquely on its exterior, where their rounded terminations, containing nuclei, may often be distinguished when the surface has not suffered abrasion. (Seefig. 416.) , The internal layer of Bivalve shells rarely presents a distinct cellular structure, when examined in a thin section ; and the residuum left after decalcification is usually a distinct but structureless membrane, closely resembling the " basement membrane" of Mr. Bowman. (MUCOUS MEMBRANE.) This form of shell substance may consequently be distinguished as membranous. In the Margaritacece and many other families, this internal layer has a na creous or iridescent lustre, which depends (as Sir D. Brewster has shown#) upon the stria tion of its surface with a series of grooved lines, which usually run nearly parallel to each other. As these lines are not obliterated by any amount of polishing, it is evident that their presence depends upon something peculiar in the texture of this substance, and not upon , any mere superficial arrangement. When a piece of nacre is carefully examined, it be comes evident that the lines are produced by the cropping-out of larninm of shell situated more or less obliquely to the plane of the sur face. The greater the dip of these laminae, the closer will their edges be ; whilst the less the angle which they rnake with the surface, the wider will be the interval between the lines. When the section passes for any dis tance in the plane of a lamina, no lines will present themselves on that space. And thus the appearance of a section of nacre is such as to have been aptly compared by Sir J. Herschel* to the surface of a smoothed deal board, in which the woody layers are cut per pendicularly to their surface in one part, and nearly in their plane in another. Sir D. Brewster appears to suppose+ that nacre con sists of a multitude of layers of carbonate of lime alternating with animal membrane; and that the presence of the grooved lines on the most highly-polished surface is due to the wearing away of the edges of the animal la minm, whilst those of the hard calcareous la minw stand out. If each line upon the na creous surface, however, indicate a distinct layer of shell-substance, a very thin section of mother-of-pearl ought to contain many thou sand laminw, in accordance with the number of lines upon its surface; these being frequently no more than 1-7500th of an inch apart. But when the nacre is treated with dilute acid, so as to dissolve its calcareous portion, no such re petition of membranous layers is to be found : on the contrary, if the piece of nacre be the product of one act of shell-formation, there is but a single layer of membrane. The mem brane is usually found to present a more or less folded or plaited arrangement ; but this has generally been obviously disturbed by the disengagement of carbonic acid in the act of decalcification, which tends to unfold the plaits. There is one shell, however,— the well-known Haliotis splendens,—which affords us the opportunity of examining the plaits in and thus presents a clear demonstra tion of the real structure of nacre. This shell is for the most part made up of a series of plates of animal matter, resembling tortoise shell in its aspect, alternating with thin layers of nacre ; and if a piece of it be submitted to the action of dilute acid, the calcareous portion of the nacreous layers being dissolved away, the plates of animal matter fall apart, each one carrying with it the membranous residuum of the layer of nacre that was applied to its inner surface. It will usually be found that the nacre membrane covering some of these horny plates will remain in an undisturbed condition ; and their surfaces then exhibit their iridescent lustre, although all the calcareous matter has been 2•e moved from their structure. On looking at the surface with reflected light under a magnifying power of 75 diameters, it is seen to present a series of folds or plaits more or less regular; and the iridescent hues which these exhibit are often of the most gorgeous description. But if the membrane be extended with a pair of needles, these plaits are unfolded, and it covers a much larger surface than before ; and the iridescence is then completely de stroyed. This experiment, then, demonstrates that the peculiar lineation of the surface of nacre (on which the iridescence undoubtedly depends, as first shown by Sir D. Brewster), is due, not to the outcropping of alternate layers of membranous and calcareous matter, but to the disposition of a single membranous layer in folds or plaits, which lie more or less obliquely to the general surface.