6 Unhcrsity of Michigan 



than do ten times that number of some small species such as 

 Amnicola limosa. In addition, the larger forms must actually 

 play a larger part, considering their actual numbers, in any 

 habitat than do the smaller species. In order to obviate this 

 difficulty in part, all of the species are divided into three 

 groups: the Unionidae, the larger (primary) species, and the 

 smaller (secondary) forms. No dominance, in the botanical 

 sense, is to be construed from this division, and the different 

 groups are purely relative. 



Table I. Habitats of the Lakes (i-n) 



* m — from marl deposits only. 

 t d — dead shells only. 



