OF NORTH AMERICA. 



75. 



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. 3 



How much of a factor these Quaternary Lakes may have been 

 in providing a haven for the Lymnseas 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 ap press a, caper at a, 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. 



FIG. 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 LYMN^EID 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 Lymnseas are pre-eminently lake and pond inhabiting 

 organisms and the habitats thus provided for them have been utilized 



3 See Chapter IV, page 82, for further discussion of this region; see also- 

 Monograph U. S. Geol. Surv. I, 1890. 



