628 Bulletin American Museum of Natural History [Vol. XLVI 



habitat, both through their lack of adaptation to the increasing aridity 

 and through the pressure exerted on them by their better adapted deriva- 

 tives. This accordingly parallels on a small scale the large-scale process 

 of climatic evolution of faunae elaborated by Matthew (1915). On the 

 theory that this has been a long-continued and fairly uniform process, 

 the composite character of the fauna may be quite simply explained. 

 The numerous species confined to the Colorado Desert proper and ex- 

 tending only into the northeastern part of Lower California represent the 

 most recent faunal development, i.e., are the most recent element in 

 the Lower Californian fauna. The only genus peculiar to this fauna is 

 Uma, which is closely restricted in habitat. A slightly older element 

 consists of the genera confined to the Colorado and adjacent desert 

 areas, which reach southern LoAver California; these are Dipso-saurus, 

 Saaro mains, Callisaurus, Phyllorhynchus, and Chilorneniscus. With the 

 exception of Phyllorhynchus, they have developed a distinct species in 

 the Cape district. The desert species which range into southern Lower 

 California without differentiation may represent the intermediate stage 

 between these two elements. A "highway" for their immigration seems 

 to be present in the Vizcaino Deserts and the Magdalena Plain. Corre- 

 lated with the development and spread of this strictly desert fauna is 

 the distribution of the species which in some sense border the desert. 

 Scaphiopus conchii, Hyla arenicolor, Phrijnosoma solare, Rhinocheilus 

 lecontei, Hypsiqlena ochrorhynchus ochrorhynchus, and Trimorphodon 

 lyrophanes, which are apparently absent from the Colorado Desert but 

 present in Lower California, reappear in Arizona at the border of the 

 Mexican Highland (Physiographic nomenclature of Fenneman, 1916, 

 p. 34). Natrix valida, which probably ranges into the State of Sonora 

 in Mexico, may be included with this list, and, somewhat more doubt- 

 fully, the fresh-water turtle, Pseudemys ornata nebulosa, which is sub- 

 specifically distinct in Lower California. A number of other species 

 which border the Colorado Desert on one side only are in full accord with 

 the distribution of the above, Utamearnsi, Xantusia vigilis, and Lichanura 

 roseofusca, being examples. In the light of the discontinuity of the ranges 

 of the foregoing list of species, it seems logical to interpret the oldest 

 element in the fauna in the same way. This element consists of genera 

 absent in the strictly desert areas to the north but present in the Cape 

 district or the San Diegan district and reappearing in southern Arizona 

 or farther south in Mexico, such as Gerrhonotus, Ctenosaura, Plestiodon, 

 Bipes, Elaphe, and Ta?itilla. The section of the genus Uta with an entire 

 frontal {Via nigricauda and microscutata) belongs here, and probably the 



