STRUCTURE OF SHELLS. 207 



All shells are composed of two portions, the one 

 consisting of particles of carbonate of lime, the 

 other having the character of an animal substance, 

 and corresponding in its chemical properties either 

 to albumen or to gelatine. The mode in which 

 these two constituent parts are united, as well as 

 the nature of the animal portion, differs much in 

 different kinds of shell ; and it is chiefly in refer- 

 ence to these circumstances that shells have been 

 divided into two classes, namely, the membranous 

 and the poi^cellaneous shells. 



In shells belonging to the first of these classes, 

 the carbonate of lime is united with a membranous 

 substance deposited in layers, which may be sepa- 

 rated from one another, either by mechanical divi- 

 sion with a sharp instrument, or by the slow actions 

 of air, water, or other decomposing chemical agents. 

 The shells of the limpet, of the oyster, and of almost 

 all the larger bivalve mollusca which reside in the 

 ocean are of this kind. They are usually covered 

 with a thick outer skin, or epidermis; and their 

 texture is of a coarser grain than that of other shells. 



If a shell of this description be immersed in an 

 acid capable of dissolving carbonate of lime, such 

 as the muriatic or nitric acids properly diluted, at 

 first a brisk effervescence is produced ; but this 

 soon slackens, and the carbonate of lime contained 

 in the shell is slowly dissolved ; the membranous 

 layers being left entire, and sufficiently coherent to 

 retain the figure of the shell ; but, having lost the 

 earthy material which gave them hardness, they 

 assume their natural form of soft and flexible plates. 



Many membranous shells exhibit, on several 

 parts of their internal surface, a glistening, silvery, 



