WHITE] ANCESTRAL ORIGIN OF UNIONID/E 87 
forms which are only modifications of the names of the living forms 
which they so closely resemble. One cannot doubt that further dis- 
coveries will yield additional evidence of the prototypal relationship 
of the fossil and living Unione faunas of this continent. 
Following the Laramie in the order of time and of geological 
sequence, are the Eocene, Miocene and Pliocene Tertiary formations, 
all three of which, in the great interior region of North America, 
consist of fresh water lacustrine deposits. From the fact that the 
Laramie Group has been found to contain so many prototypal exam- 
ples of the North American Unionide one might naturally expect to 
find among the Tertiary molluscan faunas numerous species of Unio 
that would, by similar prototypal features connect the Laramie 
forms more or less directly with the living Mississippi River fauna. 
Such, unfortunately, is not the fact, for only a few species of the 
Unionide have been found in any of those Tertiary deposits, and 
they are all of simple type and plain surface. If only such plain 
forms of Unio really existed in those Tertiary waters between the 
Laramie period and the present time, my assumption of the ances- 
trally prototypal character of the Cretaceous Uniones would be 
unsupported. Without any exception known to me, however, the 
strata in which the Tertiary Uniones have been found show evi- 
dence of having been deposited in comparatively still lacustrine 
waters, and it is a well known fact that one rarely, if ever, finds 
any other than plain types of the living Unionidz in the still waters 
of lakes. The more diverse and ornamental forms of living Uniones 
occupy fluvatile, or other running or moving waters. None of the 
deposits containing the Tertiary Uniones referred to gives any in- 
herent evidence of having been formed in fluvatile or estuarine 
waters, but such deposits were doubtless made somewhere in the 
tributaries, and upon the borders, of those Tertiary lakes. When 
such deposits are discovered they will doubtless be found to contain 
North American prototypal forms, such as will connect the Creta- 
ceous types with those of the Mississippi River fauna. 
When referring in a previous paragraph to the diverse views 
which have prevailed among naturalists concerning the present geo- 
graphical distribution of the Unionidz, it was intimated that any 
discussion of this question ought to have reference to the fossil 
Unione faunas of the respective regions. I have shown what I 
regard as good evidence that the well known types of North Amer- 
ican Uniones in the fauna of the Mississippi River have descended 
genetically from North American fossil faunas; but I am not yet 
prepared to offer an explanation of the geographical distribution of 
the Unionidz in the various regions of the world. 
