CHECKLIST OF THE SNAKES OF MEXICO 9 



Those that are restricted to Central America are relatively small 

 genera (2, 6, 2, 7, 2 species, respectively). Those that reach South 

 America vary in species content from ''. to about 51, average 13. Only 

 1 genus, of a very ancient group {Typhlops)^ occurs in the Old World. 



It is noteworthy that all the Mexican genera that occur in the East- 

 ern Hemisphere are either very ancient {Typhlops and Leptotyphlops, 

 presumably among the most primitive of all living snake genera) or 

 are essentially Holarctic and do not extend south of Costa Rica; 

 except for the first two, they are either relatively recent or generalized 

 genera. 



The highest average number of species per genus is attained by 

 those genera that occur both north and south of Mexico ; however, this 

 may well be exceeded by those extending only southward (the largest 

 group) as the South American species become better known. 



The above discussion may be summarized as follows : 



Average 

 Number of 

 Oenera Number Species 



Endemic (Nearctic 7, Neotropical 7) 14 1.6 



Practically endemic (northwestern desert only) 5 3 



Mexico and northward only (Nearctic or Holarctic) 11 10 



Mexico and southward only (Neotropical or world-wide) 30 13 



Mexico and northward and southward (Neotropical 8, Nearctic 



10, world-wide 1) 19 21 



Of more geographic significance than the distribution within politi- 

 cal boundaries is the correlation within the major biogeographic areas. 

 Thus segregated, 35 genera can be considered purely Neotropical, hav- 

 ing no or very slight extensions into the Nearctic realm. Likewise, 

 18 are purely Nearctic, while 25 overlap the 2 realms to a notable 

 degree. 



Of the 25 overlapping genera, 12 {Leptotyphlops, Constrictor, 

 Coniophanes, Drymarchon, Drymohius, Ficimia, Geophis, Leptodeira, 

 Oxybelis, Rhadinaea, Trimorphodon, and Micrurus) are very clearly 

 primarily Neotropical, having by far the greatest bulk of their range 

 south of Nearctic borders. It is remarkable that only 6 genera 

 {Opheodrys^ Pituophis^ Salvadora, Natrix, Storeria^ Agkistrodon) 

 represent the Nearctic in the same way — i. e., have their range largely 

 north of Neotropical boundaries. The remaining 7 genera {Elaphe^ 

 Hypsiglena, Lainpropeltis, Masticophis, Tantilla, Thamnopkis, Crota- 

 lus) overlap both realms very broadly. The latter group can be broken 

 down still further, however, into genera having a preponderance of 

 Nearctic species (6) and a single genus {Tantilla) having more forms 

 in the Neotropical realm. Thus only 31 genera in Mexico are clearly 

 Nearctic in origin or distribution (or both), as against 47 that are 

 Neotropical. The conclusion that the fauna of Mexico has been varied 

 to a greater degree by resurgent southern genera than by genera that 



