490 



MOLLUSKS : ACEPHA1.S. 



known representative. There are about three hundred 

 living and one thousand fossil species. Ostrca and Pcctcn, 

 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 ; //, hinge ligament ; s, sinus occupied by retractor of siphons ; p t 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. 



Pectett irradians. Lam. About one half. 

 From Cape Ann southward. 



Myiilus ednlis, L. Grows larger. 

 Both shores of the Atlantic. 



