490 



MOLLUSKS: ACEPHALS. 



known representative. There are about three hundred 

 living and one thousand fossil species. Ostrea and Pecten, 

 Fig. 451, are the prominent genera. 



Names of the parts of a Bivalve Shell. 



a, anterior retractor ; a', posterior retractor ; t, lateral teeth ; c, cardinal tooth ; /, lunale ; 

 , umbo ; ft, hinge ligament ; s, sinus occupied by retractor of siphons ; ^, pallial im- 

 pression. 



AVICULID.E, OR PEARL-OYSTER FAMILY. This Fam- 

 ily embraces acephalous mollusks which have the valves 

 unequal and very oblique. They inhabit tropical and 

 temperate seas, and yield the Mother-of-pearl, and the 

 Oriental pearls, so highly prized. They are found in 



Fig. 451. 



Fig. 452. 



Pecten irradians, Lam. About one half. 

 From Cape Ann southward. 



Mytilus edulis, L. Grows larger. 

 Both shores of the Atlantic. 



