15 



abundant distribution in both the Mississippi and Ohio valleys. 

 They may have, and their general distribution in lower Mich- 

 igan, would seem to indicate that they did make use of both 

 of the DesPlaines and Maumee outlets in their invasion of the 

 lake region. 



Two of them, U. circulus Lea and glans Lea, are purely 

 Ohio Basin forms and their occurrence in southwestern Mich- 

 igan is probably to be attributed to some former connection 

 between the head waters of the St. Joseph river and the 

 northern tributaries of the Wabash, rather than to an incur- 

 sion through the Maumee outlet and a round-about journey 

 into the western streams by means of the Huron outlet. The 

 fact that neither of these forms have been reported from the 

 interior waters of the state, would seem to corroborate this 

 view. 



In regard to the fact that a considerable number of species 

 have not been reported from the interior of the southern part 

 of the state, it is quite possible that more thorough and ex- 

 tended connections may give them a wider range than they 

 now seem to have. If, however, on further investigation this 

 fact should be established, the reason for it will probably be 

 found in some local conditions, such as, perhaps, the shallow 

 waters of the interior streams, which are unfavorable to their 

 development. 



Coming finally to the consideration of the first question 

 raised in this discussion, viz : the apparent restriction of so 

 large a part of the Mississippian portion of our fauna to the 

 Grand-Saginaw valley and that part of the state lying south 

 of it, as has already been stated, there is no apparent reason 

 in the present condition of things, why these species should 

 not have made their way into the northern waters of the state. 

 The explanation, then, must be sought for, if at all, in con- 

 ditions existing prior to or contemporaneous with the forma- 

 tion of the present topography of the state. 



As has already been stated there is reason to believe that 

 the immigration of the southern Unionidce followed very 

 closely upon the receding of the ice-cap. The southern out- 

 lets of Lakes Maumee and Chicago must have become estab- 

 lished almost simultaneously with the formation of the lakes 

 themselves, and this done, the way was open to the invaders. 



The Unionid<x of this country are essentially inhabitants 

 of comparatively shallow water, and it was, no doubt, along 

 the margin of these glacial lakes that the immigrants gradu- 

 ally spread, after they had passed through the rapidly flowing 

 waters of the Maumee and DesPlaines outlets. The beach 



