172 



THE MICROSCOPE AND ITS REVELATIONS. 



transparence; the prisms being then seen (Fig. 385) to be themselves 

 composed of a very homogeneous substance, but to be separated by definite 

 and strongly marked lines of division. When such a lamina is submitted 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. 386); 

 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 perpen- 

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

 of their length (Fig. 387); and they are frequently seen to be marked by 



FIG. 385. 



FIG. 386. 



Section of Shell of Pinna, take.ii transversely to 

 the directions of its prisms. 



Membranous basis of the same. 



delicate transverse striae (Fig. 388), closely resembling those observable 

 on the prisms of the enamel of teeth, to which this kind of shell struc- 

 ture 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 residuum will exhibit the same resem- 



FIG. 387. 



FIG. 388 



Section of the shell of Pinna, in the 

 direction of its prisms. 



Oblique Section of Prismatic Shell-substance. 



blance to the walls of prismatic cells viewed longitudinally, and will be 

 seen to be more or less regularly marked by the transverse striae just 

 alluded to. It sometimes happens in recent, but still more commonly in 

 fossil shells, that the decay of the animal membrane leaves the contained 

 prisms without any connecting medium; as they are then quite isolated, 



