LYMNAIDZ OF NORTH AMERICA. res 
the ice closely, the subarctic forms following as conditions became 
favorable, and the more temperate fauna occupying the area near the 
edge of and a little to the north of the maximum extension of the ice 
cap. 
Briefly considered, the dispersal of the family from the four areas 
outlined above was doubtless about as follows: 
(1) As the ice receded in the Mississippi Valley (figure 3), several 
large bodies of water were formed which drained into the Mississippi 

TGs Se 
Glacial Lakes in Upper Mississippi Valley, showing early outlets into 
Mississippi River. The Glacial Lake Agassiz is shown at a later. stage than 
lakes Duluth, Chicago and Maumee. (Compiled from Leverett and Upham.) 
River—one at the west end of Lake Erie, draining through the Maumee 
and Wabash rivers; one at the southern end of Lake Michigan, drain- 
ing through the Desplaines and Illinois rivers; and one at the west end 
of Lake Superior, draining through the St. Croix River. A small lake 
(Jean Nicolet) is believed to have formed southwest of Green Bay, 
