OF NORTH AMERICA. 



71 



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 



FIG. 3. 



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, 



