C. A. White—Fresh-water gill-bearing Mollusks. 385 
growth of the North American continent, especially the eleva- 
tion of its great mountain systems and plateaus, it would be 
hav 
physical features; that many of them are older than the 
mountain ranges of the regions which the rivers traverse, and 
that they have not yielded their “right of way” when the 
mountain ranges and plateaus were raised, but continued dur- 
ing and after that elevation to run in essentially the same lines 
ch t 
which they osen when the region they traversed was a 
plain instead of a mountainous one. That ancient river sys- 
tems have been in some and aes S many instances, to a 
the view concerning the general persistent integrity of rivers 
and river systems which has been referred to.* 
The coalescence of separate minor drainage systems by the 
gutacass of their lower portions into a common channel dur- 
ing the progressive ae of the continent has also been an 
important means of the dispersion of seeNrshane Mollusca. By 
age for the principal part of the continent. The Ohio and 
Upper Mississippi, the two most ancient portions of the present 
luscan fauna, which now so strongly characterizes them, until 
after the confluence with them of the western portions of the 
* The discovery of so few traces of concurs pian s have been made among 
the strata of the earth is ni due to the per: sistent adherence of rivers to 
oy ) If the lan 
been so generally i case in the gradual ‘pioduction of the North American conti- 
nent, the pea Ape r deposits were swept away in later times by their own 
waters, as their valleys were broadened and deepened. It is therefore, as a ru rule, 
only in the deposits - ror yess portions of ancient river systems that their 
faunz have been pres 
