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 Lymnzas. 
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.' 
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.’ (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.* 
(2) The recession of the ice opened the valleys of the upper 
Mississippi and the Columbia rivers and enabled the Lymnzas 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 
1See Coleman, Maury, etc., op. cit. 
2See Fairchild, Glacial Waters in Central New York, plate 35, etc. 
3See Upham, Glacial Lake Agassiz, Mon. XXV, U. S. Geol. Surv. 
