PRISMATIC SHELL-SUBSTANCE. 



595 



Fig. 307. 



their length (Fig. 307); and they are frequently seen to be marked 

 by delicate transverse stria? (Fig. 308), closely resembling those 

 observable on the 

 prisms of the Enamel 

 of teeth, to which 

 this kind of shell- 

 structure may be con- 

 sidered as bearing 

 a very close resem- 

 blance, except as re- 

 gards the mineraliz- 

 ing ingredient. If 

 a similar section be 

 decalcified by dilute 

 acid, the Membranous 

 residuum will exhibit 

 the same resemblance 

 to the walls of pris- 

 matic cells viewed 

 longitudinally, and Section of the Shell of Pinna, in the direction 

 will be seen to be of its prisms, 



more or less regularly 



marked by the transverse stria? just alluded to (Fig 308). It 

 sometimes happens in recent, but still more commonly in Fossil 



Fig. 303. 



Oblique Section of Prismatic Shell-substance. 



Shells, that the decay of the animal membrane leaves the contained 

 Prisms without any connecting medium : as they are then quite 

 isolated, they can be readily detached one from another ; and each 

 one may be observed to be marked by the like striations, which, 



Q Q 2 



