184 BULLETIN 187, UNITED STATES NATIONAL MUSEUM 

 BOTHROPS YUCATANICUS (Smith) 



Trimeresurus yucatanicus Smith, Zoologica, vol. 26, pt. 1, 1941, pp. 62-63. 



Type.—V.S.'^M. No. 46571. 

 Type locality. — Chichen Itza, Yucatan. 



Range. — Northern and eastern portions of the Yucatan Peninsula 

 (known in Mexico only from several localities in the state of Yucatan) . 



Genus CROTALUS Linnaeus 



Crotalns Linnaeus, Systema naturae, ed. 10, vol. 1, 1758, p. 214. — Glotd, Spec 

 Publ. Chicago Acad. Sci., No. 4, 1940, pp. 79-249, pis. 6-31. 



Vropsophus Waglee, Natiirliches System der Amphibien . . . 1830, p. 176 (type, 

 V. triseriatus Wagler). 



Urocrotalon Fitzingee, Systema reptilium, 1843, p. 29 (type, Crotalus durissus 

 Linnaeus). 



Aploaspis Cope, Proc. Acad. Nat. Sci. Philadelphia, vol. 19, 1867, p. 310 (type, 

 Caudisona lepida Kennicott). 



Aechmophrys Coues, Wheeler's Rep. Geogr. Geol. Expl. Surv. W. 100th Mer., 

 vol. 5, Zool., 1875, p. 609 (type, Crotalus cerastes Hallowell). 



Haploaspis Cope, Proc. Acad. Nat. Sci. Philadelphia, vol. 35, 1883, p. 13 (emenda- 

 tion of Aploaspis Cope). 



Genotype. — Crotalus horridus Linnaeus. 



Range. — "North and South America and adjacent islands from 

 southern Canada to northern Argentina and southern Brazil, exclu- 

 sive of the tropical rain forests of the Amazon River and its tribu- 

 taries." »« 



Species. — Forty-two species and subspecies; 32 occur in Mexico. 



KEY TO MEXICAN FORMS OF CROTALUS "' 



1. Outer edges of supraoculars not extended into raised, pointed, or flexible 



hornlike processes 2 



Outer edges of supraoculars conspicuously extended into raised, pointed, 

 and flexible hornlilse processes ; dorsal scales strongly keeled and 

 tuberculate; ground color usually cream, pale yellowish brown, or 

 light gray; middorsal series of subquadrate brown blotches incom- 

 pletely bordered with black cerastes laterorepens 



2. Tip of snout and canthus rostralis not markedly raised into a sharp 



ridge ; no median vertical light line on rostral and mental ; markings 



on tail, if present, in transverse rings 3 



Tip of snout and canthus rostralis raised into a sharp ridge; rostral 

 and mental usually with a median vertical light line** on a reddish 

 brown ground ; tail pattei'n terminating in longitudinal stripes, not 

 transverse bands willardi 



3. Second supralabials in contact with chin shields ; number of caudals 



less than 40 in males, 35 in females ; rattles moderate to large (more 

 than 3.5 mm. wide in adults) ; tail not extremely attenuated 4 



*' Ranges of rattlesnakes in quotation marks are from Gloyd, op. cit. 

 »' Adapted from Gloyd, op. cit., pp. 20-30. 

 " Usually lacking in specimens from Mexico. 



