206 A. G. Wetherby—Distribution of Fresh-water Mollusks, 
over regions of elevation. For these reasons, and for others of 
convenience in this discussion, I shall designate them as Fauna 
A, and will add this important and distinctly proven state- 
ment: that they reach, on our continent, their maximum o 
size, of differentiation, and the greatest local number of so- 
called species, in precisely that portion of it having the greater 
number of lakes, in regions of the oldest land or contiguous to 
it, and where there is the greatest paucity of other mollusks. 
less and interminable confusion. Nor is this statement an 
exaggeration, when we remember that European malacologists, 
of greater or less repute, have made nearly one hundred syn- 
onyms for the A. cygnea alone; and that the slightest review 
of our North American species, in the light of the evidence 
offered by geographical varieties, now well known, must reduce 
the number of so-called species more than one half; and many 
of these varieties continue from EKastern New York to Minne- 
sota, and a fewer ifumber, southward to the very borders of 
Mexico, over all of which area I have traced then! These shells, 
for like reasons with the first, I shall designate as Fauna B. 
The region occupied by A and B contains very few repre- 
sentatives of the Strepomatide, or FaunaC, Their geographical 
range northward was set forth in the first of these papers; an 
it is a significant fact that the few species of the Strepomatide, 
occupying this region, are those belonging to types that farther 
south, where the conditions of variation enumerated in another 
part of this paper reach their maximum, are so intimately 
united by varieties as to render their separation into distinct 
species, in most cases, utterly impossible; as the shells from 
different localities are so completely blended, that it is no 
exaggeration to gay that fifty per cent of the deseribed species 
are the merest synonyms. At the north, even, the difficulty 
begins; and it vastly increases in the more southerly mountain- 
ous region. This fauna differs essentially from A and B, 1p 
that it is not, normally, lacustrine, but fluviatile. A very few 
species are found in lakes, occasionally ; but there is in these / 
