WHITE] ANCESTRAL ORIGIN OF UNIONIDE 79 
effected by changes in the direction of drainage, caused by physical 
changes in the land surface, seems to be applicable to certain cases, 
but it is of course not of general application. For example, the 
Upper Mississippi and the Red River of the North have a closely 
similar Unione fauna, although the one empties into the Gulf of 
Mexico and the other into the Arctic Ocean. Their head waters 
are now far apart, and the land surface between them has only slight 
elevation. This suggestion is pertinent to the hypothesis which I 
shall present concerning the survival of the Unionide through suc- 
cessive geological periods. 
Only one more of the suggestions that have been offered in expla- 
nation of the manner in which the present distribution of the 
Unionide has been accomplished will be noticed. Indeed, this one 
is of so improbable a character that it is presented only to show 
what extreme views have been held upon this subject. This sug- 
gestion is that the Unionide of every river system which they in- 
habit originated independently from somewhat similar molluscan 
forms that existed in marine waters near the river mouths, that 
those mollusks entered the rivers, acquired the characteristics of the 
fresh water family, and differentiated into new species-and genera. 
If one should consider this suggestion seriously it may be remem- 
bered that many of the rivers which contain closely similar Unione 
faunas flow into arctic and tropical seas respectively, and that the 
molluscan faunas of those seas are correspondingly different. Also 
that some of the rivers which contain species of the Unionide and 
other fresh water gill-bearing mollusks flow into inland seas, the 
character of whose waters is such that no molluscan life can exist 
in them. Such, for example, as the Jordan, flowing into the Dead 
Sea, and the Bear and Utah rivers flowing into Great Salt Lake. 
The primary origin of fresh water mollusca from certain marine 
forms that became land-locked in local waters which gradually 
freshened as the surrounding land was elevated above sea-level, is of 
course admitted. But that a distinct and well characterized family 
of fresh water mollusks could have originated from among incon- 
gruous marine faunas at a multitude of distinctly separated centers, 
and entered the.rivers by self migration, is not to be accepted as a 
rational proposition. . 
The attempts that have been made to explain the manner in which 
the present distribution of the Unionidz has been accomplished are 
not only defective from a biological point of view, but none of them 
has had special reference to fossil Unione faunas. I shall presently 
show what I regard as good evidence that certain North American 
