O. A. White—Fresh-water gill-beariny Mollusks. 383 
great physical changes that have taken place within its area 
ing air-breathers, nothing has appa- 
rently Reapiite to prevent their safe migration to other ground 
whenever that which they may have at any time occupied 
Keane auaabtigenstal by reason of physical changes, because as 
a rule, those changes were effected so slowly that a continuity 
of congenial — for such mollusks was not neces eae? bro- 
ken. They were thus apparently as capable of preserving a 
continuous eXistenves through successive geological erie as 
the marine mollusca were. 
But, as before poatget when we come to the study of pa 
fossil melts of the fresh-water gill-bearing mollusea, which i 
their living state hae necessarily have been confined to davies 
tile and lacustrine waters, it is not easy to understand, without 
been preserved (a hey were preserved even down to 
e present time) thro a succession of geological periods, 
during which the great ee of the Laramie and succeedin 
Tertiary periods, as we know, and all the rivers which flowed 
into and from them, as is “peberilly but erroneously believed, 
ave been successively obliterated.* separate 
from each other by intervening lan , Tunning to the sea, 
But if it can be shown that throughout those geological periods 
and down to the present time there has been direct continuity 
of fresh water by means of lakes or rivers, or both, the case is 
circumstances of such vast physical changes as are known to 
h urred, we are forced to conclude that it is in this direc- 
tion shag we must seek for an explanation of the manner in 
which were preserved the fresh-water oi aR types that 
rt meri 
rivers, flourishing in the lakes, when they existed, as well as in 
the rivers, and escaping by the streams which were the former 
be suggested that the distribution of these forms from one river or 
lusks or their eggs by aquatic birds ile such trans ion is admit to 
have possible in some ot be admi robable cause of 
y 8 ble part of the di on that must have premio gents 
se ogi 
have existed. Notwithstanding the annual migration of my: ace fs gare bite 
between the northern and southern portions of North America ever since it has 
been a rteseanass the fresh-water molluscan faunze of those regions, roapactively, 
are still distin 
