72 THE CHICAGO ACADEMY OF SCIENCES. 



draining by way of the Wisconsin River. These drainage areas formed 

 a natural means of migration for the mollusks, several of which con- 

 tinued until the Mohawk and St. Lawrence outlets were opened. The 

 connection between Lake Chicago and Lake Saginaw contributed not 

 a little to the eastward dispersal of the Lymnaeas. 



A small area of dispersal possibly existed in the northern part of 

 Pennsylvania in the headwaters of the Genesee River; the Alleghany 

 River was probably used as an avenue for the northeasterly dispersal 

 of Mississippi Valley forms, although the inter-glacial invasions are 

 believed to have been from the north and west, by way of the Great 

 Lakes, during the formation to Lake Algonquin. 1 



The majority of the Mississippi Valley species now found in the 

 St. Lawrence drainage undoubtedly reached this area during this period 

 and established the present molluscan fauna of the finger lakes of New 

 York, from whence, via the Mohawk River outlet to the east and the 

 Chemung-Susquehanna River outlet to the south, they invaded the 

 eastern part of New York and Pennsylvania. 2 (Figure 4.) The glacial 

 lake Passaic, in New Jersey, probably contributed to the general dis- 

 persal. 



In the upper part of the Mississippi Valley a gigantic lake was 

 formed, known as Lake Agassiz, which occupied the territory now 

 embraced by Lake Winnipeg and adjacent lakes and the valley of the 

 Red River of the North. (Figure 3.) The outlet was at first by way 

 of the Minnesota River and thence into the Mississippi. This lake 

 provided an additional means of migration for the mollusks from the 

 lower Mississippi Valley. 3 



(2) The recession of the ice opened the valleys of the upper 

 Mississippi and the Columbia rivers and enabled the Lymnseas to 

 occupy these drainage areas, thus supplementing the work carried on 

 in the lower Mississippi Valley. 



(3) The Yukon River probably afforded the principal means of 

 dispersal from Alaska, from which area a large part of the region lying 

 west of the Canadian Rocky Mountains was doubtless supplied. 



(4) The driftless area in Wisconsin undoubtedly contributed to 

 some extent in re-establishing the molluscan fauna in the englaciated 

 territory. This area was doubtless like that immediately south of the 

 ice invasion, and the ice-bordered lakes and the numerous streams 



*See Coleman, Maury, etc., op. cit. 



2 See Fair-child, Glacial Waters in Central New York, plate 35, etc. 



3 See Upham, Glacial Lake Agassiz, Mon. XXV, U. S. Geol. Surv. 



