SUBSTANCES OF SHELLS. 105 



Many shells have projections : some like thorns, 

 others have ridges, and others rounded protuberances. 

 Those which have spines are said to be spinous ; the 

 ribs, which are longitudinal, rounded sutures formed at 

 the various growths of the shell, are called varices, from 

 the Latin for a swollen vein ; and those having rounded 

 projections are called tuberculous. 



All shells are composed of particles of carbonate of 

 lime and of what has the character of an animal sub- 

 stance, resembling in its chemical properties either albu- 

 men or gelatine. The mode in which they are united, 

 as well as the nature of the animal portion, differ much 

 in various kinds of shell ; and these coverings have, in 

 consequence, been divided into two classes — the mem- 

 braneous and porcellaneous shells. 



The two substances of which shells are composed may 

 be separated from each other by an easy chemical experi- 

 ment. If, for instance, a sufficient quantity of nitric 

 acid, considerably diluted either with water or spirits 

 of wine, be poured on a shell, or the fragment of one, 

 contained in a glass vessel, it will soon exhibit a soft 

 floating substance, constituting the animal part of the 

 shell, and consisting of innumerable net-like membranes, 

 retaining the exact figure of the shell. They satisfac- 

 torily prove that this substance is, in fact, an appendage 



