No. 418.] BASELEVELING. 847 
Thus, Io and Schizostoma are inhabitants, the first of the 
Tennessee and branches, the second of the Coosa, and neither 
of them are elsewhere found." This is an excellent illustra- 
tion of how the migration of a divide may favor differentiation. 
Fic. 5.— 
Map showing ancient (broken line) and present (continuous line) drainage in the vicinity 
of Chattanooga. (Adapted from Hayes, '99.) 
The family has thus been broken up into two parts, both 
geographically and biologically. | 
The genus Pleurocera is credited with eighty-four species, 
twenty-three of Which are found in Tennessee, and twenty-one 
ie apparently peculiar to it. This same genus is represented in 
Alabama system by fourteen species, and three of them are 
Peculiar to it. «These species,” says Tryon, “are generally 
