The loss of the original boundaries of the cells, and the consequent obscuration of the real nature of the structure, are by tic) means peculiar to shell ; for the physiologist is familiar with this change as occurring in various other tissues. Thus, in dentine, the cases in .which the vestiges of the original cells are preserved are few in proportion to those in which they are obliterated ; and yet these isolated examples are sufficient to mark the real nature of the transformation of the soft dentinal pulp into the dense ivory. It would seem as if, in the process of calcifica tion, the cell-walls have a tendency to liquefy or dissolve away, unless supported by addi tional deposits of' animal matter, thus allow ing the complete fusion of their contents. The peculiar tenacity of the decalcified shell substance in the Margaritacew and certain other tribes seems due, not so much to the strength of the original cell-walls, as to the interposition of an intercellular substance between them. In Perna we not unfre quently find, between the calcified layers, membranous lamina consisting chiefly of horny matter interposed between rounded cells that are more or less widely separated from each other : here the animal substance would seem to be peculiarly abundant, being apparently of the same kind as that of which the byssus of these animals is composed.
The ordinary account of the mode of growth of the shells of Bivalve Mollusca„—that they are progressively enlarged by the deposition of new laminm, each of which is in contaft with the internal surface of the preceding, and extends beyond it,—does not express the whole truth ; for it takes no account of the fact that most shells are composed of two layers of very different texture, and does not specify whether both these layers are thus formed by the entire surface of the mantle whenever the shell has to be extended, or whether only one is produced. An examina tion of fig. 417. will clearly show the mode in which the operation is effected. This figure represents a section of one of the valves of Unio occidens, taken perpendicularly to its surface, and passing from the margin (at the right hand of the figure) towards the umbo (which would be at some distance beyond the left). This section brings into view the two substances of which the shell is composed ; traversing the outer or prismatic layer in the direction of the length of its cells, and passing through the nacreous lining, which is seen to be made up of numerous laminm, separated by the lines aa', bb/, cc', &c. These lines evidently indicate the successive formations of this layer ; and it may be easily shown, by tracing them towards the umbo on the one side, and towards the margin on the other, that at every enlargement of the shell its whole interior is lined by a new nacreous lamina, in immediate contact with that which preceded it. The number of such laminm, therefbre, in the oldest part of the shell, in dicates the number of enlargements which it has undergone. The outer or prismatic layer of the growing shell, on the other hand, is only formed where the new structure projects be yond the margin of the old ; and thus we do not find one layer of it overlapping another, except at the lines of junction of two distinct formations. When the shell has attained its full dimensions, however, new larainm of both layers still continue to be added ; and thus the lip becomes thickened by successive for mations of prismatic structure, each being applied to the inner surface of the preceding, instead of to its free margin. The satne ar rangement may be well seen in the Oyster; with this difference, that the successive layers have but a comparatively slight adhesion to each other.
The shells of Tereb2aluice, and of several 'other genera of Brachiopoda, or Palliobran chiate Bivalves, are distinguished by peculiari ties of structure, which serve to distinguish them from all others. When thin sections of them are microscopically examined, they present a very peculiar texture, (fig. 418. a.) which might be referred either to long flattened cells, or to plications in the shell-membrane ; on the other hand, the natural internal surface of these shells always exhibits an imbricated aspect of great regularity (b). If the section
pass very obliquely towards this surface, it becomes evident that these imbrications are formed by the outcrop of the long flattened cells or folds, which were seen when the plane of the section has passed in the direction of their length. A great variety of appearances is presented by this structure, according to the direction in which it happens to be tra versed by the section ; but they are all in dicative of its peculiar character, which is readily recognisable even in the minutest fragment, although its nature yet remains doubtful. The cells, if cells they be, must be excessively flattened ; and no vestige of them can be traced in the decalcified shell ; whilst, on the other hand, the membranous residuum does not give any distinct indication of having been plicated with the regularity necessary to produce such a remarkable appearance. When any recent species of Terebratula is examined, save Ter. psittacea (which is now generally excluded from the genus on other grounds), an additional peculiarity is observed ; con sisting of the presence of a large number of perforations in the shell, generally passing obliquely from one surface to the other, and terminating internally by an open orifice (fig. 418.), whilst on the exterior they are covered in by the periostracum. Their diameter, which is greatest towards the external surface, varies in different species from about 1-1800th to 1-500th of an inch ; and there is a con siderable difference, also, in their degree of proximity to each other. In some fossil spe cies, as Ter. bullata, the interval between the passages is scarcely greater than the diameter of the passages themselves. When a portion of one of these shells, which has been pre served with the animal in spirit, has been eompletely decalcified by the action of dilute acid, the membranous residuum presents a very remarkable structure, no vestige of which is seen in the ordinary bivalves. Attached to the membranous films are a series of tubular appendages, corresponding in diameter to the perforations of the shell, and arranged at the same distances (fig. 419.) : the free extrerni ties of these appendages have distinct ccecal terminations ; and when a sufficient ing power is employed, it is found that their contents are distinctly cellular, resembling the cells in the interior of glandular follicles. These ccecal tubuli lie in the perforations of the shell, and open on its inner surface ; but there does not appear to be any system of tubes or canals for collecting the matter poured out from them, each ccecum having its distinct and independent termination on the internal surface of the shell. The surface of the mantle in contact with the shell is found to be studded with minute cells, cor responding in size and aspect with those con tained in the ccecal tubuli. The physiological purpose of this curious structure is at present a mystery ; but there can be little doubt that it is a very important one in the economy of the animal, when we see the shell thus ren dered subservient to the special protection of the ccecal appendages. The perforations are wanting in a large proportion of the very numerous species of fossil Terebratuke ; and there would appear strong reason for regard ing their presence or absence as a character of fundamental importance in the subdivision of this important genus.* In most of the non. perforated species, the shell is readily divisible into thin micaceous plates, which exhibit the characteristic texture of the shell in great perfection ; and as this texture undergoes remarkably little change in the act of fossil isation, it is often possible to recognise a Tere bratula from a very minute fragment, imbedded even in the palzeozoic strata. A very similar structure exists in several genera allied to Terebratula ; and in some of these, also, as Orthis and Spirifer, the distinction has to be established between the perforated and non perforated species ; whilst in Atrypa (to which the recent Ter. psittacea properly belongs), all the species are destitute of perforations.