ON THE MICROSCOPIC STRUCTURE OF SHELLS. 5 



until it possesses a high degree of transparency ; and it is then seen, 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 

 (fig. 3). In general the substance forming the prisms is very transparent, 

 but here and there is seen an isolated prism, usually of smaller size than the 

 rest, which presents a very dark appearance, even in a section of no more 

 than 1 -400th of an inch in thickness, as if the prism contained an opake 

 substance (fig. 6). These dark cells are seen in very great abundance, when 

 we examine a lamina in which the natural external surface has been pre- 

 served, the reduction of its thickness having been effected by grinding down 

 the under side only ; and it is then seen that their degree of opacity varies 

 considerably (fig. 5). To the cause of this appearance I shall presently revert, 

 as it is a matter of some interest in reference to the formation of this kind of 

 shell-structure. 



6. When a piece of the shell of Pinna has been submitted to the action of 

 dilute acid, the carbonate of lime being dissolved away, a consistent and 

 almost leathery membrane remains, which exhibits the prismatic structure 

 just as perfectly as does the original shell ; the hexagonal division being seen 

 when either of its surfaces is examined, and the basaltiform appearance 

 being evident on the inspection of its edge. No resemblance can be stronger 

 than that which exists between a layer of this membrane and a corresponding 

 layer of the pith or bark of a plant, in which the cells are hexagonal prisms. 

 In many instances I have been able to detect distinct nuclei or cytoblasts in all 

 the cells of a naturally thin layer ; although, from some cause which I am not 

 able to explain, these are generally invisible (fig. 8). I have often been able 

 to detect them with 7-e^ecferf light, however, when I could not distinguish them 

 with transmitted. As the nucleus occupies one of the ends of the prismatic 

 cell, it is of course useless to look for it when the natural surface of the 

 lamina has been removed by grinding. The decalcified membrane presents 

 no trace of the opake cells just now mentioned ; indeed the small cells which 

 would probably have presented this appearance in a section of the shell, are 

 now, if anything, rather more transparent and free from colours than the rest. 



7. The action of dilute acid having thus enabled us to obtain the mem- 

 branous element of shell in a separate state, we are enabled to inquire into 

 the condition of the calcareous element, by means of specimens, in which the 

 animal matter has been removed by the long-continued action of water. I 

 am indebted to Mr. S. Stutchbury for an interesting specimen, in which the 

 thick outer layer had become disintegrated 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 sepa- 

 rated by a touch. On treating these prisms with dilute acid, I have found 

 them encircled by an extremely delicate membranous film ; the remainder of 

 the cells in which they were originally formed having been removed by decay. 

 In the fossil Pinnce of the oolite and neighbouring formations, it very fre- 

 quently happens that the prisms exhibit a similar tendency to come apart, so 

 as to admit of separate examination. It is then seen, that whilst some of 

 them are truncated at both ends, so that their extremities appear at the two 

 surfaces of the layer which they form, others gradually come to apoint at one 

 end, so that this is lost in the thickness of the layer (figs. 9-1 1 ). A careful 

 examination of these prisms, and of their irregularities of form, quite disproves 

 the idea that their shape is due to a prismatic crystallization of carbonate of 

 lime, it being evident that they are casts of the interior of organic cells, the 

 shape of. which is determined by their mode of origin and formation. The 

 variations in the size of the prisms at different parts of their length, accounts 



