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es Wetherby—Distribution of Fresh-water Mollusks. 203 
Art. X VIII.—On the Geographical Distribution of certain Fresh- 
water Mollusks of North America, and the probable causes of 
their Variation; by A. G. WETHERBY, Professor of Geology 
and Zoology, University of Cincinnati.* 
Havine set forth, in the first part of this paper, the main 
facts connected with the distribution of the Unionide and 
Strepomatide,+ over the region under consideration, it now be- 
comes 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 continent's development, and 
especially with that part relating to the evolution of its 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 first fresh-water forms were lacustrine. 
Of the truth of this proposition there seems to be ample evi- 
dence in the fact, that, even during Archean times, fresh-water 
akes were not impossibilities or even improbabilities. The 
processes by which salt water areas, isolated from the main — 
ocean, pass through their various stages of approach to fresh- 
water conditions, are familiar to all students of physical geog- 
raphy ; nor is the fact of the existence of such bodies of water 
In areas of limited drainage any less well known. High pla- 
teaus and low plains alike contribute examples of this fact. 
They are most typical 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, through- 
out the geological ages, depressions of this description; and 
Rs : : 
_ when we consider the fossil shells found in lacustrine deposits, 
and the forms now inhabiting such bodies of water as Lake 
Baikal and Lake Balkash, the probability of their gradual dif- 
ferentiation from marine types, and of their successive varia- 
ions as fresh-water forms, seems to be associated with no factor 
of the improbable. 
Inthis consideration due weight must be given to the great 
* From the Journal of the Cincinnati Society of Natural History, July, 1881; 
being Part ii of an article of which Part i appeared in January, and was notice 
on p. 76 of this volume. 
t The Strepomatide comprise the American species formerly referred to the 
Melania family. 
