CHECKLIST OF THE SNAKES OF MEXICO 7 



species and geographic races remain to be discovered and defined. It 

 is not impossible that at some time the total number of snake popula- 

 tions in Mexico recognized by name may reach 600. 



At the present time 79 genera are known in Mexico. Fourteen of 

 these are completely restricted tcf that country: 



Adelophis Oeatractus Rhadinella 



Chersodromus Manolepis Symphimus 



Conopsis Procinura Sympholis 



Exelencophis Pseudoficimia Toltica 



Oeagras Pseudoleptodeira 



It may be observed that all these genera occur on the North American 

 side of the Isthmus of Tehuantepec; none extend into the Central 

 American side of the Isthmus. All the genera are rather small : 10 

 are monotypic, 3 contain 2 species each, and 1 contains 6 forms (species 

 and subspecies). All the genera are colubrids, and as such are prob- 

 ably of more recent origin than some of the more widespread genera of 

 the Pythonidae, Boidae, Typhlopidae, and Leptotyphlopidae. The 

 most ancient subfamily of the colubrids, the Sibynophiinae, is not rep- 

 resented. On the other hand, there is only one representative of the 

 presumabl}^ most recent subfamily, the Natricinae, of the colubrids, 

 and none of the more highly modified (recent?) families Elapidae, 

 Hydrophidae, and Crotalidae. Seven of the genera {Chersodromus, 

 Exelencofhis, Geagras, Geatractus, Manolepis, Pseudoleptodeira, 

 Rhadinella) appear to be of southern origin and affinity ; all these are 

 coastal or foothill genera. The relationships of Sympholis and 

 Symphimus are dubious. The remainder, all plateau genera, appear 

 to be at least of northern affinity, if not of northern origin also. 



Five other genera are practically endemic, and curiously enough, 

 all of them are restricted to northern (generally northwestern) Mexico 

 and southwestern United States. They may be considered as adapta- 

 tions to the semiarid environment of this region and accordingly do 

 not extend far onto the plateau of Mexico or far northward into the 

 United States. They are : • 



Lichanura Chionactis Micruroides 



CMlomeniscus Oyalopion 



One genus is monotypic, one is known by four forms, while each of the 

 other three contains three forms. All these may be considered rather 

 ancient in their own subfamilies or in comparison with other families, 

 and for this reason the slightly higher average number of forms per 

 genus, as compared with the endemic genera, is of interest. 



Eleven other genera, not known to occur south of Mexico, are more 

 widely distributed in the United States : 



