CONCHIFERA. 



219 



body, which protruding through the skin, give it the ap- 

 pearance and property of a rasp, which adapts it to rasp away 

 soft materials, like chalk and sandstone. In the Saxicava 

 rugosa this flinty rasp occupies the whole of the fore-part of 

 the mantle. When the little mollusk steadies itself with its 

 foot in a hollow, or attaches itself by its byssus, it obtains 

 all the support requisite, while it works away upon the 

 hard material. Some authors affirm that these Molluscs 

 prepare their cells chemically, by dissolving the carbonate 

 of lime, by means of some acid, either carbonic, acetic, 

 or lactic acid. 



A remarkable quality which some of these animals 

 possess, here deserves notice, on account of the import- 

 ance to mankind of its products. In the flesh of some 

 kinds there are often found concretions of carbonate of 

 lime, which assume very regular forms and smooth sur- 

 faces; and in those species that show a mother-of-pearl 

 lining to their shells, these little bodies have the same 

 beautiful appearance; in fact they are the pearls which 

 fetch so high a price as ornaments. There are two genera 

 from which this costly product is obtained ; the sea pearl 

 mussel (Avicula margaritiferd), which yields the so-called 

 oriental pearls, and the fresh-water pearl mussel (Marga- 

 ritana (unid) margaritiferd), in which pearls are found, but 

 they are by no means so much prized as the first. 



As food both for man and animals, the conchifera are of 

 far greater importance than all the other tribes of molluscs, 

 and the number of individuals belonging to each kind is 

 also commonly much greater. 



Touching the systematic arrangement of this class, the 

 views of naturalists are, if possible, more various than in 

 the distribution of the rest of the Mollusca. Most writers 

 divide them into those having one closing muscle (Monomy- 



