156 Cincinnati Soctety of Natural History. 
ON THE GHOGRAPHICAL DISTRIBUTION OF CERTAIN 
FRESH-WATER MOLLUSKS OF NORTH AMERICA, AND 
THE PROBABLE CAUSES OF THEIR VARIATION. 
By A. G. WETHERBY, 
Prof. Geology and Zoology, University of Cincinnati. 
PART II. 
Having set forth, in a previous number of this JouRNAL, the main 
facts connected with the distribution of the Unionide and Strepoma- 
tide, over the region under consideration, it now becomes my task to 
attempt a solution of some of the problems thereby indicated; for to - 
the careful student of this subject, several of its features are in the 
nature of unanswered questions, and these, it seems to me, will be 
found to be so intimately associated with the history of our continental 
development, and especially with that part relating to the evolution of 
the systems of drainage, as to cause continual reference to that subject, 
in the light of present geological knowledge. 
Without stopping, at this point, to discuss the zoological relationships 
which possibly indicate the marine ancestry of the mollusks under 
consideration, it is a fair presumption that the jirst fresh water forms 
were lacustrine. 
Of this proposition there seems to be ample evidence in the fact, that 
even during Archean times, fresh water lakes were not impossibilities 
or even improbabilities. The processes by which salt water areas, 
isolated from the main ocean, pass through their various stages of ap- 
proach to fresh water conditions, are familiar to all students of physical 
geography; nor is the fact of the existence of such bodies of water in 
regions of limited drainage, any less well known. High plateaus and 
low plains alike contribute examples of this fact, They are most typi- 
cal in regions of comparative aridity from various causes; and many 
such bodies of water now known, have been undergoing the freshening 
process since the early Tertiaries. 
It can not, I think, be doubted that there have been, throughout the 
geological ages, depressions of this description; and when we consider 
the fossil shells found in lacustrine deposits, and the forms now inhab- 
iting such bodies of water as Lake Baikal and Lake Balkash, the 
probability of their gradual differentiation from marine types, and of 
their successive variations as fresh water forms, seems to be associated 
with no factor of the improbable. 
