20 



THE NAUTILUS. 

 FIG. 1. 



While it is, perhaps, probable that the whole of Lake Superior 

 should be included in this system, I have hesitated to do so on ac- 

 count of the apparent failure of Unio complanatus, which may be 

 considered the characteristic species of the fauna, to extend into the 

 western part of the lake. It is quite possible that it may, but we 

 have no definite record of its occurrence west of Marquette county, 

 Michigan, on the south shore, or of the Michipicoten River, on the 

 north shore. It was not found by the University of Michigan expe- 

 ditions of 1904 and 1905, either in the streams of Ontonagon county, 



