FRESH-WATER MOLLUSKS. 571 



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

 sisting 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. 



B 



^C30 O 



Fig. 10.^, Pearly Concretions from a Fresh-water Mussel; B, Pearly Concretions from the 



Commou Oyster. 



In shells having a brilliant, pearly lining, or nacre, the pearls ob- 

 tained are oftentimes very beautiful, and from certain 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. These 

 kinds of pearls are often found in oysters. 



The Chinese have long been familiar with the art of making arti- 

 ficial pearls. By partly opening the shells of certain fresh-water mus- 

 sels, and inserting little lead images, or other objects, between the 

 mantle and the shell, the objects soon become covered with a natural 

 layer of pearl. 



Let us now study the markings on the inner surface of the shells of 

 river-mussels. The shells of these creatures are called valves, and are 

 spoken of as right or left valves, according to whether they are on the 

 right or left side of the animal. 



Certain ridges and prominences will be seen at the hinge, and, when 

 the valves are carefully joined, the ridges in one valve will correspond 

 to grooves in the other valve. These ridges are called teeth. The 

 short ones, near the beak, are called cardinal teeth, and the long ones 

 lateral teeth. The margin upon which they occur is called the hinge- 

 margin, for it is upon this margin that the valves turn. {See Fig. 11.) 

 Certain scars, or impressions, will be found marking the inside of 

 the valves, and these indicate the point of the attachment of certain 

 muscles to move the valves, and to enable the animal to protrude its 

 foot, and crawl along. These marks are hence called muscular marks, 

 or muscular impressions, and will be found to correspond in the right 

 and left valves. 



An irregular, round impression will be found at each end of the 

 valve, near the hinge-margin. These show where the muscles are at- 

 tached to move and close the valves, and hold them firmly together. 

 The muscles run directly across from one valve to the other; and, to 

 open a live mussel, it is necessary to pass a sharp blade between the 



