2 LAND AND FRESH WATER MOLLUSKS 



the North, part of the channels which earlier discharged into 

 the Gulf of Mexico by way of the Mississippi. It is probable, for 

 the region under review, that the entire moUuscan population 

 was exterminated or driven south during the Glacial epoch, and 

 that we now have to do with immigrants from the south whose 

 distribution has taken place since that time. 



In the following tables all doubtful species are omitted, so 

 that the fauna tabulated, if not complete, is at least well estab- 

 lished so far as it goes. 



The following areas are represented in the successive columns 

 of the table : 



Ast'a. — This column indicates those species known also to 

 inhabit the eastern portion of Siberia, Kamchatka, and the 

 Chukchi Peninsula, together with the Commander Islands, 

 which are obviously populated from the Asiatic shores. 



Yukon. — This system includes the entire drainage basin of 

 the Yukon and the tundra north of it, as well as the area drained 

 by the Kuskokwim River, or all of Alaska north, northwest, 

 and westward of the Alaskan Range, as well as the area behind 

 the Coast Ranges and between them and the northward exten- 

 sion of the Rocky Mountains, drained by the Yukon and its 

 tributaries. 



Alaska. — This system includes all of the Aleutian Islands, 

 the area on the Alaska Peninsula and continent between 

 the Coast Ranges and the Alaskan Range and the Pacific 

 north of latitude 54°. This system and the following one are 

 really continuous, the Alaskan being really only the north- 

 westward extension of that here designated as the Pacific 

 system. 



Pacific. — This includes the coast drainage of British Colum- 

 bia, the basins of the Fraser and Columbia rivers, the coastal 

 part of the State of Washington, and the northern part of Idaho 

 and Montana west of the Selkirk Range and its more southern 

 equivalents in the Rocky Mountain region. 



Mackenzie. — This vast system includes the basin drained by 

 the Mackenzie River and its tributaries, covering northwest 

 Alberta, northeastern British Columbia, the northwestern two 

 thirds of Athabaska, and the Mackenzie district. 



