THE LIFE OF GLACIAL LAKE CHICAGO 83 



South of Green Bay record 



Wisconsin, between Prairie du Chien and De Soto 38 80 miles 



Minnesota, not recorded l 



Iowa, Lansing 39 80 " 



Michigan, not recorded 



Illinois, Utica, LaSalle Co. 40 220 " 



Ohio, Scioto River 41 260 " 



Indiana, Tippecanoe 42 230 " 



It is interesting to note that recent crassidens is found only in the Mississippi 

 River drainage. 43 



It has been thot that the presence of this species so far north might indicate 

 a period during which a warmer climate than the present prevailed. The 

 accompanying species, however, are mostly of northern distribution, and this 

 assumption may rest upon insufficient grounds; it may be a case in which the 

 mollusk was not able to adapt itself to a new invironment and so became 

 extinct so far as these regions are concerned. The lowering of the water, 

 changing the environment from a bay to a marsh may have caused the extinc- 

 tion of the Chicago colonies. The Green Bay fauna evidently migrated up the 

 Mississippi and Wisconsin rivers and thence thru an embayment of glacial 

 Lake Chicago, the precursor of the present Green Bay. 



Pleurobema coccineum magnalacustris is also of special interest. It is 

 greatly inflated and differs quite markedly from the typical form of coccineum. 

 Simpson remarks that it "seems almost entitled to specific rank. " It is found 

 in the St. Lawrence drainage, particularly near Niagara Falls, which is the 

 type locality. The typical form of coccineum is not represented in the Chicago 

 deposits, but the variety is one of the most abundant of naiads next to crassidens 

 and pustulosa. The shells mentioned by Miss Letson from the gravels of 

 Niagara Falls, are probably this race. 44 As it is not now found in the Mississippi 

 drainage, it must have become extinct in this region during one of the stages 

 of Lake Chicago. Typical coccineum is rare or wanting in the St. Lawrence 

 drainage; the race magnalacustris seems to have migrated via the Chicago 

 outlet to the vicinity of Niagara Falls where it flourished thru several lake 

 stages, but finally became extinct in all but a few localities. Recent specimens 

 have been seen 45 from the Detroit River and the Grand River, Michigan, 



38 Chadwick, Bull. Wis. Nat. Hist. Soc, rV, p. 95, 1906. 



39 Museum record. 



4 » Baker, Bull. HI. State Lab. Nat. Hist., VII, p. 77, 1906. 



41 Sterki, Proc. Ohio Acad. Sci., IV, p. 392, 1907. 



42 Daniels, 27th An. Rep. Dept. Geo!., Indiana, p. 650, 1902. 



43 The deposits in and about Green Bay should be carefully examined, for they doubtless 

 contain much material bearing on the question of life distribution during postglacial times. 



44 Geology of Niagara Falls, p. 252, figure 190. 

 43 Walker collection. 



