386 0. A. White—Fresh-water gill-bearing Mollusks. 
present great river system which brought that fauna from its 
ancient home in the western part of the continent. 
Rivers having been thus persistent, and the manner in 
which confluence of the waters of many of them has been 
effected being understood, it is no more remarkable that the 
types of fresh-water gill- bearing mollusca have come down to 
us be baean teh be, than it is that marine and land mol- 
lus hed us bearing the imprint of their really 
ancient but “WAAE we have been accustomed to call, modern 
Not only have the molluscan denizens of the great Missis- 
sippi drainage system descended to their present habitat in the 
manner suggested ; but there is no reason to doubt that a large 
part of the fishes of that system descended in the same manner, 
and in company with them. This is thought to be especially 
true of the characteristic Ganoids of that system. The progeni- 
tors of many of the fresh-water fishes may have aspanded from 
the sea by the mouths of the rivers which have since coalesced 
to form that great river system; but it is believed that all did 
not do so 
Both cetbiniaa and ichthyic life doubtless began in the sea ; 
and it seems at least probable that the freshwater denizens of a 
large part of both these classes became such by compulsion 
h : 
lu t 
Mississippi drainage system have come down wholly un changed 
from a time at least as remote as the Laramie Period. 
