CONSTRUCTION OF SHELLS. 3 
ceous coatings perform the offices of bones, by 
furnishing to the tendons that fixed basis, without 
which, mechanically, they could not act. All this 
“most strikingly evinces a carrying on of the same 
plan. 
But how, it may be asked, are the shells of testa- 
ceous animals constructed? They appear of two 
descriptions, with regard to the substances of which 
they are composed. 
I. Shells with a porcelainous aspect, and enamelled 
texture, such as the Volute and Cowry. These, when 
submitted to the test of experiment, evidently consist 
of carbonate of lime, cemented by a very small por- 
tion of animal jelly. 
II. Shells, furnished generally, if not always, with 
a strong epidermis, which is principally composed 
of the substance called mother-of-pearl. The Oyster 
and River Muscle, furnish familiar examples. Shells 
of this kind differ in their composition from the 
preceding only by possessing a smaller proportion 
of carbonate of lime; which, instead of being simply 
cemented by animal gluten, or jelly, is intermixed 
with, and serves to harden a membranaceous or car- 
tilaginous substance; and this substance, even when 
deprived of the carbonate of lime by infusion in 
diluted nitric acid, still retains the figure of the 
shell. 
B2 
