76 GASTEROPODA. 
solidity and hardness. The chemical composition 
of these shells, however, varies very little; they 
consist of carbonate of lime deposited in cells of 
animal albumen. In the porcelain shells, of which 
the Cowries (Cyprea) afford familiar and beautiful 
examples, the lime is compact, with so small a 
portion of animal matter, that when immersed in 
acid the shell is completely dissolved, no sensible 
trace remaining. In the pearly shells, such as the 
genus Zrochus, the calcareous matter is deposited 
in layers; and these, when submitted to acids, leave 
behind an insoluble membrane of albumen which 
retains the form of the shell.* 
The shell is secreted by the mantle. In one 
family, that of the Chitons, it consists of several 
pieces, but in general it is simple, and takes the 
form of a hollow cone produced in various degrees. 
In the Limpets, which we see adhering so abun- 
dantly to our sea-side rocks, the cone is low and 
nearly symmetrical; but in the great majority of 
this class, the cone is greatly lengthened and twisted 
upon itself, so as to form a spire. 
The mode in which the shell is formed has been 
well investigated by Mr. J. EX. Gray, whose obser- 
vations on the subject I shall here take occasion to 
cite :— 
“The shell, which is peculiar to this division of 
the animal kingdom, may be seen covering the 
young animal in the egg before it has gained all its 
organs, as was observed by Swammerdam, and 
verified by the more extended observations of 
Pfeiffer, Turpin, and others. They are easily seen 
in the egg of the Limnat, Physe, Ancyli, and 
Bithinie, which have a transparent coat. 
* Thomson’s Chemistry, v. 54. 
