48 C. A. White—Fresh-water and Land Mollusca. 
Reviewing the collections which represent the fossil faune 
herein discussed, so many familiar forms are seen that it is dif- 
ficult to realize the fact that a large proportion of them, includ- 
ing those especially which have been mentioned by name in 
this article, were living contemporaneously with the last of the 
Dinosaurs. Yet such is the fact, and the shells of the former 
are often found commingled with the bones of the latter. What 
were the successive steps in the history of the transmission of 
these types from that remote time to the present we are unfor- 
tunately without the means of knowing with certainty, because 
of the remarkable paucity of molluscan remains in all the de- 
posits of the great interior region later than the Kocene. All 
the mollusecan remains, which have been found in these later 
deposits, belong to familiar living types, although of extinct 
species. 
That the palustral and land pulmonates might have been, and 
perhaps were, preserved under immediate conditions differing 
from those which insured,the survival of the Unionide is evi- 
dent; but certain facts point to the conclusion that the peculiar 
“North American” types of Uniones which prevailed in the 
Laramie epoch were not transmitted through the Kocene, Miocene 
and Pliocene epochs as denizens of the fresh-water lakes which 
succeeded the brackish water of the Laramie sea, and each 
ime The Eocene fresh-water deposits contain a plate 
ble number of species of Unio, it is true, but they are all, so 
tute a type which, although common among living Uniones, is 
exceedingly rare if not entirely wanting in ‘the Laramie group. 
The conclusion therefore seems necessary that those peculiar 
and varied forms of Unio which have been mentioned in the 
preceding list, with their faunal molluscan associates, escaped . 
from the Laramie lacustrine waters before the close of that 
epoch, into those pokes waters which formed the outlet to 
the ses deisins and which became a part of the Mississippi 
drainage system, as the shavalioa of the continent progressed.* 
he magnitude of the physical changes which have taken 
place upon the North American continent since the epochs in 
which the Mollusca lived, eae = discussed in this bere 
* This subject is discussed at some length in Bull. U.S. Geol. Sur. Terr., vol. 
iii, p. 615. 
