12 BULLETIN 18 7, UNITED STATES NATIONAL MUSEUM 



Class REPTILIA Laurenti 



Reptilia Latjbenti, Specimen medicum exhibens synopsin reptilium . . ., 1768, 

 p. 19. 



Order SQUAMATA Oppel 



Squamata Oppel, Die Ordnungen, Familien und Gattungen der Reptilien . . ., 

 1811, p. 14. 



Suborder Serpentes Linnaeus 



Serpentes Linnaeus, Systema naturae, ed. 10, vol. 1, 1758, p. 214. 

 KEY TO THE MEXICAN GENERA 



1. Scales of uniform size about body, midventral scales not enlarged ; scale 



rows 14, 18, or 20 (possibly 24 in 1 species) around body 2 



Ventral scales larger than others, transversely elongate; scale rows 

 variable, 13 to 79 3 



2. Ocular reaching lip; scale rows 14 aroimd body Leptotyphlops (p. 20) 



Ocular separated from lip by labial scales and sometimes a subocular 



as well; scale rows 18 or 20 (possibly 24) TypMops (p. 19) 



3. Tail strongly flattened laterally for swimming ; sea snakes- Pelamis (p. 176) 

 Tail not strongly flattened, more or less rounded 4 



4. A deep pit between nostril and eye 5 



No pit between nostril and eye 8 



5. A rattle or large horny button on tail 6 



No rattle or button on tail 7 



6. Top of head covered with nine large plates Sistrurus (p. 199) 



At least posterior (parietal) head scales split into smaller scales; 



usually numerous small scales on top of head Crotalus (p. 184) 



7. Top of head covered with nine large plates Agkistrodon (p. 177) 



At least posterior (parietal) head scales split into smaller scales; 



usually numerous small scales on top of head Bothrops (p. 179) 



8. Rostral enlarged, turned up in front, keeled above; usually an azygous 



scale following rostral, sometimes other small scales Heterodon (p. 71) 



Rostral not enlarged or, if so, not keeled above 9 



9. Numerous small scales boi-dering rostral posteriorly; no enlarged plates 



on top of head ; all dorsal head scales uniformly small- Constrictor (p. 25) 

 Two to five large plates bordering rostral posteriorly, between labials ; 

 at least anterior scales on top of head enlarged 10 



10. One or two pairs of enlarged, elongate chin shields on lower jaw, with 



which at least the four anterior infralabials are in contact 13 



No enlarged scales between anterior infralabials, or in one genus one 

 small anterior pair in contact with less than four infralabials on 

 each side 11 



11. Rostral enlarged, prominent, somewhat turned up; a pair of large pre- 



frontals, in contact on each side with supralabials Loxocemus (p. 27) 



Rostral not enlarged ; no typical prefrontals 12 



12. Two large, median scales on top of head behind rostral, one following 



the other Ungaliophis (p. 26) 



No enlarged, median head scales Lichanura (p. 25) 



