PRISMATIC SHELL-STRUCTURE OF BIVALVES. 507 



The shell is thus seen to be composed of a vast number of 

 prisms, having a tolerably uniform size, an<J usually presenting 

 an approach to the hexagonal shape. These are arranged per- 



FIG. 251. 



FIG. 252. 



Fig. 251. Section of Shell of Pinna, transversely to the direction of its prisms. Fig. 252. Mem- 

 branous basis of the same. 



pendicularly (or nearly so) to the surface of the lamina of the 

 shell ; so that its thickness is formed by their length, and its two 

 surfaces by their extremities. A more satisfactory 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. 251) that 

 the prisms themselves appear to be composed of a very homo- 

 geneous substance, but they are separated by definite and 

 strongly marked lines of division. When such a lamina is sub- 

 mitted to the action of dilute acid, so as to dissolve away the 

 carbonate of lime, a tolerably firm and consistent membrane is 

 left, which exhibits the prismatic structure just as perfectly as 

 did the original shell (Fig. 252) ; the hexagonal divisions being 

 apparently the walls of cells resembling those of the pith or bark 

 of a plant, in which the cells are frequently hexagonal prisms. 

 In very thin natural laminae, the nuclei of the cells can often be 

 plainly distinguished. By making 

 a section of the shell perpendicu- 

 larly to its surface, we obtain a 

 view of the prisms cut in the di- 

 rection of their length (Fig. 253); 

 and they are frequently seen to be 

 marked by delicate transverse striae 

 (Fig. 254), closely resembling those 

 observable on the prisms of the 

 enamel of teeth, to which this 

 kind of shell-structure may be 

 considered as bearing a very close 

 resemblance, except as regards the 

 mineralizing ingredient. If a 

 similar section be decalcified by dilute acid, the membranous re- 

 siduum will exhibit the walls of the prismatic cells viewed longi- 

 tudinally ; and these will be seen to be more or less regularly 

 marked by the transverse striae just alluded to. It sometimes 



FIG. 253. 



Section of the Shell of Pinna, in the 

 direction of its prisms. 



