THE DISTRIBUTION OF THE UNIONIDAE IN ALASKA AND BRITISH 



AMERICA. 



BY BRYANT WALKER. 



(Abstract.) 



This is a preliminary and partial report on a study of the Unionidae of 

 Alaslia and British America undertaken at the request of the Canadian Biologi- 

 cal Survey, and tlie final report will be published by that Survey. 



As only a portion of the material submitted for examination by the Survey 

 has been determined, the exact number of species represented in the area and 

 the I'ange of the several species has not yet been definitely ascertained, and 

 therefore will not be discussed at the present time. 



But enough is now known of the general character of the fauna and its 

 distribution to determine its salient features and the sources from which it 

 was in all probability derived. This is the ground covered by the present 

 paper. 



North America, north of the Ohio River and west of the Mississippi River, 

 is divided, so far as its Unione fauna is concerned, into three great faunal 

 areas, known as the Atlantic, Mississippi and Pacific areas. All three are 

 represented in the area studied. 



The Atlantic fauna occupies New Brunswick, Nova Scotia and the lower 

 portion of the St. Lawrence valley, extending west as far as Lake Ontario and 

 thence northwesterly through Georgian Bay into Lake Superior. The region 

 lying north of the St. Lawrence valley and between Labrador and Hudson's Bay 

 is entirely unexplored, and its Unione affinities are wholly unknown. This 

 faima, except as remnants may have survived in favorable localities along 

 the coast during the glacial period, is wholly post-glacial, and was derived 

 from immigration from the southeast. 



The Pacific fauna is found only west of the great range of mountains 

 extending from Alaska to Mexico. Its relations are wholly with the northern 

 Eurasian fauna, and it was undoubtedly derived from some of the earlier 

 migrations over the land bridges between Asia and Alaska. 



The entire country lying between these compartively restricted areas is 

 occupied by the Mississippi fauna. Just how far north this fauna extends is 

 a most interesting question, and one that yet remains to be determined. In 

 British America the Mississippi fauna occupies two distinct areas, separated 

 by the northwestern extension of the Atlantic fauna. 



Between Lake Superior and the lower St. Lawrence the fauna of the St. 

 Lawrence valley is Mississippian, and was derived by post-glacial migration 

 from the Ohio valley. This has been discussed by Walker (Nautilus, XXVII, 

 1913). 



2l8t Mich. Acad. Sci. Kept., 1919. 



