and the probable causes of their Variations. 207 
shells an inherent aversion to still water, which characterizes 
all the genera, leading them to frequent rather the rapid parts of 
rocky streams; and here it is that we meet their greatest diver- 
sity of types, and the greatest variety of coloration and orna- 
mentation. This peculiarity of station is so persistent, that no 
skilled collector ever searches for them in level reaches of deep 
water, unless in the case of a few species of Pleurocera, which 
groups are only represented by the genus Melanopsis, over the 
same range in Kurope and Asia, and by Goniobasis and Pleuro- 
cera at the north, in America, their grand metropolis; in for- 
eastern and southeastern tributaries of the Tennessee, we find, 
as has already been stated, a group of shells of a distinct facies, 
_ requiring no expert knowledge of conchology to enable one to 
see that it differs, as a whole, from the Fauna D, with which it 
in this discussion, and in this place. In his last edition of his 
Synopsis of the Family Unionidw, 1870, which he tells us is 
his “most important work,” Mr. Lea makes the following 
remarkable statement, the truth of which he had abundant 
opportunities to verify; “although I have examined critically, 
