been filed with the Section. Anodonta i?nbecilis Say, is the 

 only species, which is not more or less burdened with 

 synonymy. 



For the purposes of this paper, therefore, I have com- 

 bined the sixteen species now recognized into six groups and 

 in this way the general results can be considered as approxi- 

 mately correct. The groups are 



1. Group of Anodonta corpulenta Cpr. x 



2. " " " grandis Lea. 1 



3. "' " " edentula Say. 



4. " " " fenissaciana Lea. 



5. " " " fragilis Lam. 



6. " " " imbecilis Say. 



Of these, four, in their various forms, are our most 

 abundant species, and have been found practically everywhere 

 where any collecting has been done. They are the groups 

 of 



Anodonta grandis Lea. 

 edentula Say. 

 fragilis Lam. 

 fenissaciana Lea. 



One species of the group of Anodonta grandis, Anodonta 

 simpsoniana Lea is a northern form, originally described 

 from the Great Slave Lake, and has not been noticed further 

 south than Houghton Lake, Roscommon county. It is, how- 

 ever, believed to be an offshoot of the parent stock "altered 

 and dwarfed" by peculiar environmental conditions, such as 

 in the case of the Unto luteolus, have given rise to the forms 

 known as Unto borean's Gray, and superiorensis Marsh, in the 

 same region. 



Anodonta corpulenta Cpr. and its allies have thus far 

 been found only in the western waters of the state. 



Anodonta imbecilis Say is a species substantially con- 

 fined to the southern portion of the state, although it has 

 been found as far north as Ludington, Mason county, on the 

 Lake Michigan shore. 



Summarizing these results, we find that of the forty 

 species of Unio, only seven, or less than one-fifth, are known 

 to range over the whole state. Three species are character- 

 istic of the northern part of the state, while thirty, or seventy- 



1 These groups in a purely systematic arrangement would probably be united. 

 Their separation here is solely to bring out their apparently peculiar distribution. 



