rilE FRESH-WATER CLAM 237 



on all coasts. Our Ostrea virgmiana ^ 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. 

 In Japan, however, there is a species occasionally gaining a 

 length of three feet. The principal European species is 

 Ostrea edulis. The oyster was formerly commoner and larger 

 north of Cape Cod than it is now, for the great aboriginal 

 shell-heaps (" Kitchen-middens ") of the New England coast 

 contain shells of enormous size. The cause of this gradual 

 extinction of the oyster is uncertain, but is believed to be due 

 partly to general climatic and geogTaphic changes and partly 

 to overfishing. 



1 Fig. 228. 



