11 



tirely Mississippian in character, only two of the sixty-one 

 species being- peculiar to the Atlantic fauna. In the north- 

 ern part of the state, however, where the total number of 

 species is greatly reduced, the proportion as a faunal element 

 of course, becomes much greater. 



Of the two species belonging to this fauna found in Mich- 

 igan, one, Anodonta ' fragilis Lam., is of universal distribu- 

 tion through the state and is usually found in great abund- 

 ance in all our inland lakes. It is extremely variable, no 

 less than six species having been established on its varieties 

 by Anthony and Currier. 



The other, Unio complanatus Sol., has until lately been 

 supposed to be confined to the northern part of the state, but 

 recently evidence has been obtained that it is found at Mus- 

 kegon, on the Lake Michigan shore, and in Macomb, Lenawee 

 and Monroe counties on the eastern side. The latter are the 

 first instances on record of the species having been found in 

 any streams flowing into Lake Erie from the south and west, 

 while the former is the only known record of the occurrence 

 in any waters tributary to Lake Michigan. 



Both of these species undoubtedly found their way into 

 the state from the north and east. In view of the uninter- 

 rupted water-ways which have existed from time immemorial 

 between the great lakes and the Atlantic Region, the route 

 of this immigration is obvious and requires no explanation. 



In regard to the existence of so large an element of our 

 fauna of Mississippi valley species, however, the case is quite 

 different. There is at the present time no natural water con- 

 nection between the great lakes and either the Mississippi or 

 Ohio river valleys. All the rivers in Michigan are tributary 

 to the great lakes and reach the ocean through the St. Law- 

 rence river. 



How then, did such an immigration of species from a 

 region now entirely separated from the great lakes occur ? 

 The explanation is to be sought for in the topographical 

 changes incident to the glacial period. 



During the glacial period, the entire state of Michigan 

 was buried for several thousands of years under an ice cap of 

 enormous thickness, which extended in some places as far 

 south as the Ohio river. This undoubtedly resulted in the 

 complete extermination of all forms of molluscan life in this 

 region, which may have previously existed. With the reced- 

 ing of the ice, great bodies of water were formed, bounded on 

 the south by the height of land and the north by the con- 

 stantly retreating glacier. No outlet being possible to the 

 eastward on account of the ice, the water made a way for 



