4r IXTRODUCTIOX TO CONCHOLOGY. 



Each shell is therefore composed of animal and calcareous 

 matter ; the first constitutes a membranaceous basis^ which 

 is equahy curious and beautiful, being either formed of cells 

 with hexagonal walls, or else of laminae, more or less wrinkled 

 like morocco leather. But however differing, they are both 

 recipients of the calcareous matter already mentioned ; the 

 lime in both cases imparting solidity to the membranaceous 

 tissue, that would otherwise be little more than a mere cob- 

 web. Procure, for instance, the shell of any specimen of 

 Pi una J and break off a small portion from the thin margin 

 of the outer layer which projects beyond the inner ; submit 

 that portion to a low magnifnng power, and when examined 

 by transmitted light it will present, on each of its surfaces, 

 very much the appearance of a honeycomb. Look narrowly 

 and you will discover further, that whilst at the broken edge 

 it exhibits a fibrous aspect, itmay be compared, in reality, 

 to an assemblage of basaltic columns. A still closer in- 

 spection will reveal tbat the shell is composed of a vast 

 number of prisms ai-ranged perpendicularly to the surface of 

 its laminse, forming the thickness by their length, and the 

 two surfaces by their extremities. And each of these, 

 although composed of very homogeneous substances, are 

 separated by strongly marked hexagonal divisions, resembling 



