rUK NALTIl-US. 99 



One of the marked topographic features of Wisconsin is a long 

 dia^ional valley extending from Green Bay, and really as a continua- 

 tion ot tlie basin of liiis bay, toward the southwest. It follows the 

 basin of Lake Winn(l)ago and the course of the Fox river. In the 

 neighborhood of Portage it overrides the water-shed, and is con- 

 tinued in tJie valleys of the Wisconsin and the Rock. At Portage 

 the Fox and the Wisconsin are less than two miles apart, and in 

 Spring become confluent, the upper Wisconsin contributing largely 

 toward the floods of tiie Fox (Irving, Geology of Wisconsin, Vol. 

 11, pp. 418, 419). 



Now, according to Irving (1. c. p. 426), it is very probable that 

 in preglacial times the entire area of the Fox river drainage, includ- 

 ing the basin of the Wolf, far north of Green Bay, was drained by 

 the Wisconsin, or a stream occupying approximately its bed. Given 

 this former unity of the Fox and the Wisconsin drainage, the oc- 

 currence of a Mississi|)pi form as a fossil in Green Bay is made clear, 

 even though this form be now a southern one. For it must be re- 

 membered here that southern forms in general had a decidedly more 

 northern distribution before the Pleistocene, and especially before the 

 Pliocene. 



How as to its disappearance ? We know that during the Pleisto- 

 cene the northern part of our hemisphere became ice-coated nearly 

 as far south as the Ohio river. One of the lobes of this great ice 

 mass entered this very same Green Bay — Wisconsin Valley — and 

 plowed through it nearly its entire length. 



It is evident that this enormous ice mass swept everything living 

 before it, or buried it beneath, and Uvio crassidens had to go with 

 the rest. 



When the ice finally receded the conditions were so changed as 

 to forbid the establishment of previous faunal conditions. In the 

 first ()lace, the drainage of the Fox was now separated (rom that of 

 the Wisconsin. But more important, the climate of this region had 

 become so much colder that many of the former inhabitants, U. 

 crassidens among them, seem not to have been robust enough to re- 

 gain even such part of their former territory, to which the waterway 

 was freely open. Finally alteration in tension between various 

 species probably also contributed to the same general result. 



It is highly desirable that the Unionids, as well as other mollusca 

 found on both sides of the divide between the Mississippi and St. 



