hd 
re 
< ° 
Geographical Distribution of Certain Fresh-Water Mollusks. 159 
of cases, it may be regarded as a substantial proof of their high anti- 
quity when taken in consideration with the following facts; first, their 
universal presence in the lakes of the older geological formations at 
the north; second, their circumpolar distribution; third, their presence 
in regions unfavorable to the development of other families of mollusks, 
as testified by their absence ; fourth, their persistent appearance to- 
gether, even southward, over regions of elevation. For these reasons, 
and for others of convenience in this discussion, I shall desiguate 
them as Fauna A, and will add this important and distinctly proven 
statement; that they reach, on our continent, their maximum of 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 re- 
gions of the oldest land or contiguous to it, and where there is the 
greatest paucity of other mollusks. This fauna is thus clearly shown 
to be regional, and the inference is fair that it has avery high anti- 
quity. 
Over the same region, both in Europe and America, we have distrib- 
uted a few species of the Unionide, mostly represented by the genus 
Anodonta, a lacustrine group, always affecting still waters with muddy 
bottoms. These forms, with plain surface, and comparatively thin 
shells, are the predominant types of this family over the whole north- 
ern portion of our continent, from Maine to Oregon. Itis among these 
mollusks that there occurs the greatest apgarent synonymy, and the sys- 
tematic zoologist will find himself, in the study of these shells, face to 
face with the question of varieties in endless and interminable confu- 
sion. Nor is this statement an exaggeration, when we remember that 
European malacologists of greater or less repute have made nearly one 
hundred synonyms 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 eastern New York to Minnesota, and a fewer 
_ number southward to the very borders of Mexico, over all of which 
area I have traced them! ‘These shells, for like reasons with the first, 
T shall designate as Fauna B. 
The region occupied by A and B contains very few representa- 
tives of the Strepomatide, or Fauna C. Their geographical range 
northward was set forth in the first of these papers; and it is a signifi- 
cant fact that the few species of the Strepomatide occupying this 
region are those belonging to types that, further south, where the 
