FRESH-WATER MUSSELS. 33 



the shell, the entire surface of the mantle deposits the pearly 

 substance which lines the inner surface of the shells, and 

 which is so characteristic of the fresh-water mussels. 



31. Grains of sand, or other particles, getting in between 

 the mantle and the shell, are soon covered by layers of pearly 

 substance poured out, or secreted by the mantle. In this way 

 pearls are formed. 



If pearls are broken open, a, centre, or nucleus, will be 

 found, consisting of some particle of dirt or sand, or some 

 substance which had found its way by accident between the 

 mantle and the shell, and around which the pearly matter 

 has been formed in successive layers. 



Fro. 38. A, Pearly Concretions from a Fresh-water Mussel; , Pearly Concretions from 

 the Common Oyster. 



In shells having a brilliant, pearly lining, or nacre, the 

 pearls obtained are oftentimes very beautiful, and from cer- 

 tain Oriental species living in the sea, called Avicula, the 

 most brilliant pearls of commerce are obtained. If, on the 

 other hand, the nacre lining the shell is dull white, as in the 

 common oyster, the pearls are dull-colored. This kind of 

 pearls is often found in oysters. 



The Chinese have long been familiar with the art of 



