No. 418.] BASELEVELING. ' 849 
to the upper part of the Coosa River, and Lioplax has two 
species, one from the Coosa and the other from the Mississippi 
valley drainage. The species Campeloma ponderosa is found in 
both river systems. 
An interesting illustration comes from crayfish. Cambarus 
spinosus, extraneous, and erichsonianus are found in both the 
Tennessee River and in the head-water parts of the Coosa- 
Alabama system. 
A head-water fauna is very different from a lowland fauna; 
thus a knowledge of the habitats of animals will furnish criteria 
by which we are able to recognize, to some degree, the condi- 
tions under which divides have migrated. Since valleys con- 
tinually lead their rapid-water fauna to the divides, these forms 
are the first which become transferred when they are shifted. 
What evidence is there to support this view? This is well 
shown if we consider those forms which are the firs? to sur- 
mount mountain barriers. 
The Rocky Mountains have been a very formidable barrier 
to our fauna, there being but few passes or gaps through them. 
The fish fauna on each side of these mountains in the United 
States are remarkably distinct, but two species, according to 
Jordan (96, p. 118), are found on both sides of this great bar- 
rier, and these species are very naturally rapid-water species, 
and not lowland forms. These two fish are the Rocky Moun- 
tain trout (Salmo mykiss Walb. and the Rocky Mountain 
whitefish (Coregonus williamsoni Gir.). In the Appalachian 
Mountains the upland living brook trout (Sa/velinus fontinalis 
Mitch.) illustrates the same point, it being found on both sides 
of the range. 
Extension of Lowland Faunas into Uplands by Baseleveling. 
River valleys are well known as highways for the dispersal 
of animals. These valleys, of. course, are not always due to 
the process of baseleveling, but in the southeastern Appala- 
chians we have good illustrations of the extension of a valley 
due to this process. The valley of the Tennessee River above 
Chattanooga is an excellent illustration of this. By means of 
