362 POPULAR SCIENCE MONTHLY. 



ON THE RELATIONS OF THE LAND AND FRESH-WATER 

 MOLLUSK-FAUNA OF ALASKA AND EASTERN 



SIBERIA. 



By WILLIAM HEALEY DALL, 



SMITHSONIAN INSTITUTION. 



"I T AVING recently completed a census of the land and fresh-water 

 -* — *- mollusk-fauna of Eastern Siberia and Alaska, together with a 

 discussion of the relations of its elements, it seemed that a summary 

 of the results in their bearing on geographical distribution of animal 

 life in the boreal regions might have some interest for the readers 

 of the Popular Science Monthly. 



In the region north of latitude 49° the molluscan population of 

 North America is rather scanty. For distributional purposes it must 

 be divided into two series, one containing the aquatic forms and the 

 other the land shells. The distribution of water animals is carried 

 on by different means from those influential in the dispersal of ter- 

 restrial forms, and any discussion which combined the two without 

 distinction would be liable to contain errors of fact and deduction. 



The vast territories under consideration have a number of drain- 

 age systems which in the tabulation of species should be distinguished. 

 The chief of these are: 



1. The Labradorian. — This comprises the area of drainage into 

 Ungava Bay and the Atlantic north of the straits of Belleisle and the 

 ' Height of Land/ including the Labrador coast and the northeastern 

 part of the Ungava District of the Dominion of Canada. 



2. The Canadian. — This system comprises the drainage of the St. 

 Lawrence and the Great Lakes south and east from the ' Height of 

 Land,' including the island of Anticosti. 



3. The Hudsonian.— -This, the largest system of all, includes the 

 entire area draining into Hudson Bay, including Keewatin, the south- 

 eastern corner of the Mackenzie District, eastern Athabaska, the 

 whole of Saskatchewan, the southeastern two-thirds of Alberta, As- 

 siniboia and Manitoba, the drainage area of the Red river of the 

 north in the Dakotas, and northeastern Minnesota, all of Ontario, 

 Quebec, and Ungava north and west of the ' Height of Land.' 



4. The Mackenzian. — This vast system includes the basin drained 

 by the Mackenzie river and its tributaries, covering northwestern 

 Alberta, northeastern British Columbia, and the northwestern two 

 thirds of Athabaska and the Mackenzie District. 



