384 C. A. White—Fresh-water yill-bearing Mollusks. 
outlets and inlets of the lakes, but which continued to flow 
after the obliteration of the latter, as rivers or tributaries of 
river systems. 
Lakes are only parts of unfinished river systems which dis- 
appear when the system is finished by the erosion of its 
~ 
channel to a nearly uniform slope. A lake consequentl 
may ae cas that at least a part of the river chan- 
nels of to-day have existed as such ion former geological 
times; that the fiaior part of them were Se in epochs 
anterior to our own, and that those of some of the tributaries 
of the present Mississippi River system are decdeat at least 
in part, with former outlets or inlets, or both, of the great 
ancient lakes which have been referred to. Consequently we 
of those ancient lakes, and the river systems of which they 
constituted lacustrine portions. This view is confirmed by the 
identity of the living with the fossil molluscan types which 
a oe n found so “abundantly i in those Laramie and Eocene 
de 
tn th these someon the ancient Laramie sea is included under 
the te rine,’ the term ‘“‘sea” being used simply to 
asae that i its nee were saline and not fresh; just as the 
Black and Caspian are called seas instead of lakes, and for the 
same reason. It may seem to be the use of a misnomer to 
speak of the Laramie sea as a part of a-river system, because 
it was so immensely large, and the continental area which was 
a, sis its tributaries and outlet, differed only in degree and 
n kind, from any river system which has a lake of any 
size in its princi ¥ co he of that sea having 
een saline, the ae eee omen more n 
ivers have x opie existed ever since a sufficient extent 
of continental surface was raised above the git to accum mulate 
d in view of ‘the 
mighty changes that have taken place Seen ‘he Gainnnaea 
