STRUCTURE OF SHELLS. 231 



that shells have been divided into two classes, 

 namely, the membranous and porcellaneous 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 separated from one another, either by 

 mechanical division with a sharp instrument, or 

 by the slow actions of air, water, or other de- 

 composing 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 pro- 

 duced, 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, sil- 

 very, or iridescent appearance.* This appear- 



: 



Examples of this nacreous structure, as it is termed, occur 

 in the shells of the Haliotis, or Sea-ear, and of the Anodon, or 

 fresh water muscle. 



