3 BULLETIN 18 7, UNITED STATES NATIONAL MUSEUM 



Three are coast-to-coast genera (Coluber, Diadophis, Natrix), 3 are 

 central and eastern (Heterodon, Opheodrys, Sistrurus), and 5 are 

 central and western. Most of the central and western genera are of 

 southern origin, while the others are to be considered northern. The 

 number of forms per genus ranges from 2 to about 25, and averages 

 10, in the Americas; 3 genera (Coluber, Opheodrys, Natrix), occur 

 in the Eastern Hemisphere, where they are represented by about 1, 4, 

 and 80 forms, respectively. 



Nineteen Mexican genera occur both north and south of the coun- 

 try. Four of these extend only a very short distance southward 

 (Guatemala) : 



Pituophis Storeria 



Salvadora Agkistrodon 



The other 15, in addition to occurring northward in the United States, 

 extend into South America or a considerable distance into Central 

 America. They are: 



Leptotyphlops Ficimia Tantilla 



Coniophanes Lampropeltia Trimorphodon 



Dnjmarchon Leptodeira Thamnophis 



Drymobius MasticopMs Mbcrurus 



Elaphe Rhadinaea, Crotalus 



The forms per genus, in the Americas, vary from 5 to 62 and average 

 about 21. Three genera occur in the Old World: Leptotyphlops, 

 Elaphe, and Agkistrodon. The first belongs to an extremely ancient 

 group and in at least this hemisphere centers in South and Central 

 America. The other two belong to very recent groups and in the 

 New World do not extend south of Costa Kica. 



The bulk of the genera (30) occur outside of Mexico only toward 

 the south. Five extend only through Central America or part of it: 



