16 



lines of these lakes, therefore, represent with substantial 

 accuracy the first territory occupied b)* the new fauna. 



The accompanying - diagram (PI. Ill) of the 100 and 

 200 levels, taken from the contour map of the state, prepared 

 by the late Dr. Alexander Winchell, in 1878, and believed to 

 be substantial^ correct, shows approximately the position 

 of these ancient beaches with reference to the present lake 

 shore of the state. 



The 200 feet level on the southeast being substantially 

 the shore line of Lake Maumee, while the 100 feet level repre- 

 sents the eastern shore of Lake Chicago and, after the open- 

 ing of the Saginaw outlet, and the consequent lowering of 

 Lake Maumee, the shores of both lakes, whose waters were 

 then on practically the same level. 



In process of time as the level, first of Lake Maumee and 

 then of both lakes, was lowered, and the present s}'stem of 

 drainage was established, the Unionidce would naturally fol- 

 low the retreating waters, and thus finally found their way 

 into the rivers on either side of the state as we now find them. 



From the line of the glacial lake beaches down to their 

 mouths the Unionidce seems to have had no difficult}' in occu- 

 pying the rivers of the southern portion of the state. But in 

 the upper waters of these streams, above that line, few species 

 seem to have been able to obtain a foot-hold. As has already 

 been stated of the large number of species which are now 

 found south of the Saginaw-Grand valley but five appear to 

 range generally across the state. The remainder are found 

 only in the lower waters of the rivers on either one or both 

 sides of the state, and in the Saginaw-Grand valley. And it 

 is certainly significant, that so far as the returns show, the 

 range of these species toward the interior, is substantially 

 coincident with the beach line of the glacial lakes. That is 

 to say that it would appear that the majority of our species, 

 which come from the south, have migrated down stream from 

 the place where they first obtained a foothold in the present 

 river systems, but for some reason, with the exception of a 

 few species, have not succeeded in spreading up stream to any 

 considerable extent. 



As the low land which had been occupied by the Huron 

 outlet was the last to emerge from the waters, so it was prob- 

 ably the last to receive its unione fauna. And until it was 

 substantially closed, the width and depth of this great channel 

 was apparently too great to be overcome by the species, which 



