THE FRESH-WATER CLAM AND ITS ALLIES 187 



The Ledidae include certain elongated shells with a 

 shiny epidermis, and a large number of teeth on the hinge, 

 which are found chiefly in mud in rather deep water. 

 The animal has a large foot 

 (Fig. 174) with which it bur- 

 rows rapidly. It can also 

 move rapidly through the 

 water as the scallops do. 



The Ostreidae 1 include the 

 oysters, which are the most 

 important invertebrate food 

 fishery, being valued at 

 twenty to thirty million dollars per year for our Atlantic 

 seaboard. Oysters are found on all coasts. Our Ostrea vir- 

 giniana 2 extends from the Gulf of St. Lawrence to the Gulf 

 of Mexico. The oysters of our Western coast are not so 

 important as the Atlantic species. 0. edulis is the prin- 



FIG. 174. Yoldia, one of the Lc- 

 didae. Foot protruding below. 

 Nat . size. Photo . by W . H. C . P . 



FIG. 175. Ostrea virgmiana, the Eastern oyster. One-third iiat. size. 

 Photo, by W. H. C. P. 



cipal European species. In Japan there is a species occa- 

 sionally gaining a length of three feet. The oyster 

 was formerly commoner and larger north of Cape Cod than 



1 From Greek name for oyster. 2 Eig. 175. 



