the Skeletons of Mollusca, Crustacea and Echinodermata. 379 



the thin laminae into which it separates, without any preparation, 

 by the aid of even an ordinary microscope. These cells are 

 usually hexagonal in form, but are not very regularly so. Their 

 size varies considerably, both as to diameter and length, even in 

 the same shell. When very long, the layer which they form 

 will possess considerable thickness ; when they are very short, it 

 will be delicately thin. The cells are usually the longest in the 

 outer layers. I have seen a recent Pinna in which one of these 

 was nearly ^th of an inch in thickness ; and I have seen this far 

 surpassed in fossil shells allied to this genus, as Inoceramus, Pa- 

 chymya, &c. ; yet in the very same shells, the inner layers were 

 thinner than a sheet of writing-paper. Although there are con- 

 siderable variations in the diameter of the cells, in different parts 

 of the same individual, and still more in different species of the 

 same genus, yet there appear to be certain limits ; so that some 

 shells may be characterized by the coarseness of their prismatic 

 structure, and others by its fineness. The coarsest I have yet met 

 with is in Inoceramus, the finest 'va. Pandora ; and the difference 

 is so great, that at least 250 cells of the latter would be covered 

 by one cell of the former (Plate XIII. figs. 1 and 2). 



The resemblance, in their structure and position, between the 

 prismatic cellular substance of shells, and the prismatic epithelium 

 covering the mucous membranes of higher animals, leaves no room 

 for doubt of their analogy ; and we may consider this form of shell- 

 structure in the light of a calcified epithelium, the carbonate of 

 lime being deposited in the cavities of the cells, and in general 

 completely filling them. In most sections of Pinna a greater or 

 less number of dark cells may be seen, which are usually disposed 

 with some degree of regularity. These I have reason to believe 

 to be cells incompletely filled with carbonate of lime ; the spaces 

 remaining in which, being occupied by air only, present a dark 

 appearance under the microscope, according to the general prin- 

 ciple well known to observers. 



I think it cannot admit of a doubt, that the prismatic arrange- 

 ment of the carbonate of lime in these shells is due to the nature 

 of the organic basis in which it is deposited, and not to anything 

 peculiar in its mode of crystallization. An interesting specimen 

 of Pinna has lately been brought under my notice by Mr. S. 

 Stutchbury, in which the thick outer layer had become disinte- 

 grated, during the life of the animal, by the decay of its organic 

 structure ; and the prisms of carbonate of lime were left in situ, 

 but not in any way held together, so that they could be separated 

 by a touch. On treating these prisms with dilute acid, I find 

 them encircled by an extremely delicate membranous film ; the 

 remainder of the cells, in which they were originally formed, 

 having been removed by decay. 



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