Geographical Distribution of Certain Fresh-Water Mollusks. 163 
sophically called ‘“ comprehensive ty pes;”’ and it is by no means a diffi- 
cult thing to show abundant evidences of their presence in this hetero- 
geneous host of their modified descendants, as I hope to point out here- 
after. Even if this fauna does not antedate the Carboniferous epoch, 
the station which it has always occupied, for reasons already shown, 
would have brought a maximum of differentiating causes to bear. 
Nothing seems clearer to me than the separate origin of D and E. 
This is indicated by the merest superficial study of the shells, and I 
confidently expect that future geological explorations, among the west- 
ern Tertiaries, may bring to light additional evidence upon this subject; 
and that when the habits and anatomy ofthese animals have been more 
thoroughly studied, and when we have a fuller understanding of the 
relations existing between the living and fossil species of western 
Kurope, and the fossil Tertiary species of southeastern Europe, new 
light will begin to break in upon the “origin of species’’ among these 
protean bivalves; for such work is the special province of geology, and 
the highest generalization to which the perfection of geological knowl- 
edge can lead us. In considering the facts connected with the ex- 
ploration of the western lake basins, we find the Unionide to be dis- 
tributed through the whole series of deposits from the Jurassic to the 
Tertiary, and well through the latter. In a very philosophical discus- 
sion of this subject, Dr. White has shown that there is an interming- 
ling of forms, and an extent of differentiation pointing to remoter 
origin. But he has, in a foot note on page 620, made the following 
statement that needs correction. “It is a significant fact that those 
North American rivers which contain the richest Unione fauna drain 
Mesozoic and Tertiary regions, while those that drain Paleozoic and 
Azoic regions have a comparatively meagre Unione fauna.’ The 
whole drainage of the Ohio is Paleozoic, or so nearly so that we may call 
it such. This stream and its tributaries south and southeast are the 
metropolis of these shells. And it is here that we find the two faunas 
above indicated most distinctly developed. The rivers draining the 
Mesozoic and Tertiary regions of the west have a very meagre fauna, 
both as to species and individuals; and I have already stated, that with 
the exception of the few Anodontas of the northwest, the entire assem- 
blage is composed of Ohio types. Until series of casts of the Ohio 
river shells are made, and these are carefully compared with the casts 
of species described from these western localities, we shall not have 
reached the best conclusion which a study of these fossils will afford 
us. If we consider the species of the Mesozoic and Tertiary regions 
