18 THE NACTILUS. 



THE UNIONE FAUNA OF THE GREAT LAKES. 



BY BRYANT WALKER, SO. D. 



The Unione fauna of the Great Lakes is one of considerable 

 interest to the student of geographic distribution. It might naturally 

 be expected that the St. Lawrence system, extending from Minne- 

 sota to the ocean, and affording a continuous waterway of more than 

 2,000 miles, and which flows nearly east and west through a region 

 of substantially the same climatic and other environmental condi- 

 tions, and with no natural connections with the Mississippi and Ohio 

 systems, would be inhabited by a common fauna, throughout its 

 entire length. As compared with the Mississippi drainage system, 

 which extends from the far north to the almost semi-tropical regions 

 of the Gulf States, it would seem that the fauna of the latter would 

 naturally be much more diverse in its character than that of the St. 

 Lawrence system, but the contrary is the case. The fauna of the 

 Mississippi Valley, from one end to the other, is a substantially 

 homogeneous fauna, varying simply in the number of species in differ- 

 ent parts of its extent. But on examining the Unionidae of the 

 Great Lakes, we find that, while the fauna of Lake Superior, at the 

 western extremity of the system is similar to that of the lower St. 

 Lawrence, and the New England States, there is in the center of 

 the system, with Lake Erie as its metropolis, an entirely different 

 fauna, which extends eastward as far as the Ottawa River and Mon- 

 treal, and westward to the Saginaw Valley, and even perhaps as far 

 as Mackinac. The relations of this fauna are entirely with that of 

 the Ohio and Mississippi Valleys. 



This interpolation of a distinct faunal area in the middle of a 

 great drainage system is very remarkable, and, so far as I know, is 

 without parallel in any other of the great river systems of the world. 

 And when, in addition to this, we find that there this intermediate 

 fauna is, in almost every case, so modified from the typical form of 

 the several species represented, that, in a very large proportion of 

 the species, the Great Lake forms have, at one time or another, 

 been described as species distinct from the typical forms as found 

 in the Mississippi fauna, and that this fact has recently been made 

 the basis of an argument by Dr. Scharff, in his interesting book on 

 the " Distribution and Origin of Life in America," for his theory of 



