34 FIEST BOOK OF ZOOLOGY. 



making artificial pearls. By partly opening the shells of 

 certain fresh-water mussels, and inserting little lead images, 

 or other objects, between the mantle and the shell, the ob- 

 jects soon become covered with a natural layer of pearl. 



32. Let the pupils 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. 39.) 



33. 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 muscu- 

 lar 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 attached 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 



