212 A. G. Wetherby—Distribution of Fresh-water Mollusks. 
able; but they were not favorable, and consequently no such 
distribution has taken place. Hence it is, that the few species 
of shells inhabiting those streams, seem to me more likely to 
be the descendants of an ancestry of old date, and their general 
correspondence in form to the Ohio type, points to their com- 
munity of origin. The Fauna E is here wanting; nor has it 
any representative. When we come to the consideration of 
the down-stream distribution of the species east of the Missis- 
sippi, we find the Strepomatide, as represented by their most 
characteristic genera, and Fauna E of the Unionide, to have a 
barrier in that direction. Here they cease, and beyond it, in 
the Tennessee, Cumberland, etc., we find mainly the Fauna D. 
Since this fact is general, it becomes one of high significance in 
this discussion, and stands as a unique evidence in favor of 
some of the suggestions here made; and it shows conclusively, 
that continuous water is not the only condition of molluscan dis- 
tribution; and that the present station of Jo, Goniobasis, Ancu- 
losa, etc., in mountain streams, and in the more rapid por- 
tions of these streams, is the result of the presence of condi- 
tions to which these creatures are by nature fitted; and while 
ies are more cosmopolitan, owing to their greater 
grea 
bull of Fauna C has its range circumscribed as has here been 
indicated. 
teristic geological groups; and to these evidences I shall direct 
attention in a future article. 
