594 



PRTSMATTC SHELL- SUBSTANCE. 



Fig. 305. 



Basaltic columns (Fig. 403, p). The Shell is thus seen to be com- 

 posed of a vast number of prisms, having a tolerably-uniform size, 

 and usually presenting an approach to the hexagonal shape. These 

 are arranged perpendicularly (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 ex- 

 tremities. A 

 more satisfac- 

 tory view of 

 these prisms is 

 obtained by 

 grinding-down 

 a lamina until 

 it possesses a 

 high degree of 

 transparence ; 

 and it is then 

 seen (Fig. 305) 

 that the prisms 

 themselves 

 appear to be 

 composed of a 

 very homo- 

 geneous sub- 

 stance, but 

 that 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 to- 

 lerably firm 

 and consistent 

 Membrane is left, which exhibits the prismatic structure just as per- 

 fectly as did the original shell (Fig. 306) ; its hexagonal divisions 

 bearing a strong resemblance to the walls of the Cells of the pith or 

 bark of a Plant. By making a section of the Shell perpendicularly to 

 its surface, we obtain a view of the prisms cut in the direction of 



Fig. 306. 



Fig. 305. Section of Shell of Pinna, transversely 

 to the direction of its prisms. 

 Fig. 306. Membranous basis of the same. 



