14 



eastern part of the state to Lake Erie and the streams im- 

 mediately tributary to it, that while two, U. hippopiCits Lea 

 and Leibii, Lea are local forms of questionable specific value, 

 which are not found elsewhere, the remaining - four are pecu- 

 liar to the Ohio Basin and are not found in the upper waters 

 of the Mississippi. 



The inference, therefore, that these species found their 

 way through the Maumee outlet and established themselves 

 in southeastern Michigan before that outlet was closed seems 

 conclusive. The fact, that they have not yet apparently 

 spread beyond the streams tributary to the Detroit river and 

 Lake St. Clair, explains why they did not effect an entrance 

 into the western part of the state through the Huron outlet 

 before that too was closed. The further fact, that one of 

 them, U.fabalis Lea, is cpnoted from the upper waters of the 

 St. Joseph river, in northern Indiana, by Call, 1 does not mili- 

 tate against this theory, but is rather to be explained by the 

 close proximity of the head waters of that stream and those 

 of some of the tributaries of the Wabash, which is pointed 

 out by that author, and a probable immigration from them 

 into the St. Joseph. 



In the same way, the fact that two of our species U. as- 

 ■perrimus Lea and cornutus Bar. are now apparently found only 

 at the extremities of the Grand — Saginaw valley ; and that 

 three others, U. lavissimus Lea, kirtlandianus Lea and tenu- 

 issimus Lea are reported from the Grand river only, would 

 seem to indicate that these species were about the last to find 

 their way into our waters before communication with the 

 M ississippi was cut off, and for that reason succeeded in estab- 

 lishing themselves only in the ancient channel of the Huron 

 outlet. 



In a like manner, the present apparent distribution of the 

 Anodonta corpulenta Cpr. and its allies in the state, would 

 seem to indicate in some degree the history of its immigra- 

 tion. The typical species is a characteristic and peculiar 

 form restricted to the upper waters of the Mississippi valley. 

 In Michigan, the group seems to be confined to the streams 

 tributary to Lake Michigan. A significant fact when taken 

 into connection with its centre of distribution. 



In regard to the remaining members of our fauna of Miss- 

 issippian descent, which have a more or less general distri- 

 bution over the southern part of the state, there is little, if 

 anything, to indicate the route taken by them in their immi- 

 gration. Most of them are common species of general and 



i Hydrographii Basins of Indiana and their Molluscan Fauna. Pro. Ind. 

 Acad. Sci. 1896, p. 247. 



