160 UNIVERSITY OF COLORADO STUDIES 
examined the manuscript, and has made a critical examination of a 
Notropis which I had failed to identify. 
In the study of geographical distribution, especially as related to 
past conditions, the value of the evidence afforded by fresh-water fishes 
can hardly be exaggerated. Thus the following contrast between the 
genera of the Gila and Rio Grande basins should make those hesitate 
who believe in the recent depression of the continent in the region of 
southern New Mexico and Arizona. 
CYPRINIDAE 
Gita R. Basin BotH Rio GRANDE BASIN 
Gila *Leuciscus (also one in *Campostoma (also one in Yaqui R. 
Tiaroga Yaqui R. basin, the species _ basin) 
Agosia of all three basins very *Hybognathus 
Meda close.) *Pimephales 
Cochlognathus 
Cliola 
*N otropis 
*P henacobius 
*Rhinichthys (but also in Great Basin) 
*H ybopsis 
* Boulder County, Colo, (Platte R. Basin). 
The case of Leuciscus, and the rather similar one of Rhinichthys, 
come under the head of ‘“‘exceptions which prove the rule;” for the 
close resemblance (in Rhinichthys even identity) of species on the 
Atlantic and Pacific slopes, taken with the great general diversity, 
simply shows that these forms must have been transported in some way 
from one basin to another since the separation of the drainage areas. 
That they are ultra-conservative forms, preserving their characters 
while all around them has changed in the course of ages, seems scarcely 
possible. * 
The general similarity between the fishes of the Rio Grande and 
Platte basins is as striking as the diversity in the other case. 
It is noteworthy that the trout (Salmo), which inhabit the waters of 
« The strong and numerous fish fauna of the Mississippi valley may be thought of as spreading west- 
ward, to be checked by the Rocky Mountain chain. How recent this spread may be, and how far it has re- 
sulted in the extermination on the Atlantic slope of the mountains of specially western types, remains uncer- 
tain; adequate paleontological evidence is wanting. It is proper to remember, however, that characteristic 
Mississippi valley types of Mollusca occur in the Cretaceous of the Rocky Mountain region. 
