13 



the slight barrier offered by the existing- watershed was over- 

 come, and the water flowed both east and west. 



Toward the close of the glacial period therefore, the 

 northern part of the lower peninsula was an island separated 

 from the main land by this great channel. 



The existence of this ancient waterway is of peculiar in- 

 terest to the conchologist because, as has already been stated, 

 it to-day apparently forms the northern limit of the range of 

 nearly three-fourths of our Unione fauna. 



There can be no doubt that it was through these ancient 

 channels that the barren waters of the lake region was peo- 

 pled by an immigration of southern forms.* 



That the more hardy and vigorous species of the Mississ- 

 ippi fauna followed closely the retreating glacier, is shown 

 by the fact that two species, Unto ellipsis Lea and liitcolus 

 Lam. have been found fossil in the ancient beaches of Lake 

 Agassiz, a glacial lake in Minnesota and Manitoba, contem- 

 poraneous with the Maumee and Chicago lakes, at heights 

 varying from 165 to 275 feet above the present level of Lake 

 Winnipeg. 1 While in the glacial drift near Toronto, Ontario, 

 three species have been found fossil, which are not known to 

 exist at the present time in the lake region. 



It has been suggested that this latter find has an "im- 

 portant bearing upon the theory of a mild inter glacial period, 

 preceded and followed by an advance of the ice. If the ice 

 receded to the vicinity of Toronto, allowing these Mississippi 

 species to attain to that region, the fact that they did not 

 establish themselves there would be easily accounted for by 

 the subsequent advance of the ice and the destruction of the 

 colony. The final melting and disappearance of the ice cap, 

 being complicated by change in the direction of the drainage, 

 might not afford a second opportunity for the immigration of 

 the species in question." 2 In view of the fact that all of 

 these species are found abundantly in the Ohio valley and 

 that one of them, U. davits Lam, is not found in the upper 

 waters of the Mississippi river, the query naturally arises, 

 whether the closing of the Maumee outlet did not have some- 

 thing to do with their failure to re-establish themselves in the 

 lake region upon the final receding of the ice. 



And, in this connection, it is interesting to notice in 

 reference to the six species which are restricted in the south- 



*. Simpson. Am. Nat XXX, p. 379, and Pro. U. S. Nat. Mus. XVIII, 

 p 338. 



1. Upham — The Glacial Lake Agassiz, page 237. 



2. Simpson— Pro. U. S. Nat. Mus. XXI., p. 595. 



