8 



five per cent., are confined to the southern portion of the 

 state and do not extend north of the Grand-Saginaw valley. 

 Of these, six are found only in the southeastern portion of 

 the state, in the streams tributary to Lake Erie. 



Of the five species of Margaritiha, two only are of 

 general distribution, the three others being- inhabitants of the 

 southern part of the state, and one of these is apparently 

 restricted to the tributaries of Lake Erie. 



Of the six groups of Anodonta, four, embracing all the 

 species but three, are of general distribution. One of the 

 remaining groups, (that of Anodonta corpulenta Cpr.), seems 

 to be restricted to the rivers immediately tributary to Lake 

 Michigan and the third, a single species, is of general dis- 

 tribution in the southern part of the state. 



In other words, taking the fauna as a whole, we find: 



1st: — That but a comparatively small number of the 

 species are of general distribution. 



2nd: — That a few are peculiar to the northern part of the 

 state. 



3d: — That a few appear to be restricted to Lake Erie and 

 the waters immediately tributary to it in the southeastern 

 part of the state. 



4th: — That a great majority (75 per cent. ) of the species 

 of Unto and Margaritina are only found in the southern part 

 of the state, and are substantially confined to the Grand- 

 Saginaw valley and that portion of the state lying south of it. 



In view of the fact that the state lies wholly in one 

 drainage area and is not traversed or cut up by any of the 

 great natural barriers, which ordinarily limit the range of 

 species and form the boundaries of zoological provinces, and 

 that in the lower peninsnlar, at least, there are no such 

 marked differences in topographical, geological or climato- 

 logical conditions in any of its parts, which would naturally 

 suggest such a radical difference in their fauna, these re- 

 sults seem strange and, if true, very curious and interesting. 



The most obvious and natural reply to these conclusions 

 is, of course, that the} 7 are not true, or rather, perhaps, that 

 the most thorough work of our collectors has been done in the 

 southern portions of the state, and that consequently, a 

 gteater number of species have been found there; that com- 

 paratively little thorough work has been done in the northern 

 portions of the state, and that in all probability, if it were 

 done, the number of species would be largely increased, and 

 that until such work is done it is entirely wrong to attempt 

 make any such generalizations at all. In short, that at the 



