LYMN-EID OF NORTH AMERICA. ion 
Lake are the descendants of Lake Bonneville, while Humboldt, Walker, 
Pyramid, Carson and other small lakes in western Nevada are the 
descendants of Lake Lahontan.* 
How much of a factor these Quaternary Lakes may have been. 
in providing a haven for the Lymnzas which were driven southward 
is not known. The connection with the Snake River might have pro- 
vided a highway for the entrance into Lake Bonneville of such species. 
as stagnalis appressa, caperata, obrussa and palustris, which now occupy 
the area of the Great Basin. The streams of this area now have no 
outlet to the sea, but either flow into small lakes or are lost in the 
desert. The Great Basin, therefore, should be separated as a distinct 
faunal region. 

EG 6: 
Portion of driftless area in Wisconsin, showing regular erosion in typical 
unglaciated country. (From map published by Geological Survey of Wiscon- 
sin. ) 
EFFECT OF THE GLACIAL PERIOD ON THE LYMNEID FAUNA OF AMERICA. 
The effect of the various ice invasions of the northern part of 
North America has been to produce a territory in which there are 
more lakes, large and small, than in all of the rest of the world com- 
bined. The Lymnzas are pre-eminently lake and pond inhabiting 
organisms and the habitats thus provided for them have been utilized 
3See Chapter IV, page 82, for further discussion of this region; see also 
Monograph U. S. Geol. Surv. I, 1890. 
