present time our knowledge of the distribution of the various 

 species belonging- to our fauna is too imperfect and incom- 

 plete to justify any attempt in that direction. 



While this criticism is undoubtedly just to a certain ex- 

 tent, and while it is true that our knowledge of the northern 

 part' of the state is far less perfect than that of the southern 

 portion, and that the work done in the north has been some- 

 what fugitive in its character, and that there are, indeed, 

 whole counties of whose fauna we know nothing, yet it seems 

 to me that there is a fair basis for reaching the conclusions 

 above stated. 



While the work which has been done in the northern 

 part of the state has been, perhaps, comparatively small, it 

 has nevertheless been pretty well spread over the entire upper 

 end of the lower peninsula. 



I have indicated upon the accompanying map (PL 1) i the 

 known range of two of our most common species, Unto 

 luteolus Lam. and Anodonta footiana Lea, which experience 

 shows are to be found practically everywhere. 



This shows, therefore, exactly where work has been done 

 and how much of the territory has been covered. I have also 

 shown upon the same map the range of Unio rubigmosus 

 Lea, which the reports show to be one of the most abundant 

 species in the southern part of the state. 



Other species could be added if it were necessary, but 

 these will, I think, be sufficient for the purpose. It will be 

 noticed that in the southern part of the state the range of the 

 three species is substantially the same, but that, while in the 

 northern portion the two species of general distribution still 

 continue to be found together, the southern form has disap- 

 peared entirely. 



Now these species are all about of the same size and are 

 what would be called good-sized species and are not likely to 

 be overlooked by the average collector. 



Moreover, the work in both parts of the state has been 

 done by the same persons, so that the difference cannot be 

 laid to any personal equation on the part of the collectors. 



It seems very improbable, therefore, that if the southern 

 forms did extend into the northern part of the state, they 

 would have been so completely overlooked by the same 

 collectors, who had no difficulty in finding them in the south. 

 If this were the case of a few species only, the present 

 fragmentary condition of our knowledge on the subject might 

 be considered an adequate reason for the discrepancy. But 

 where, as here, it exceeds to seventy-five per cent, of the 

 whole fauna, it does not seem to me to be a satisfactory ex- 

 planation. 



