4 REPTILES. 



3. Parias. Superciliaiy shield single, hinder labial shields 

 smallest ; scales lanceolate, of head and body smooth, of crown 

 unequal. Asia. 



4. Meg^ra. Superciliary shield double, hinder labial shields 



smallest ; scales lanceolate, of head and body smooth. Asia. 



5. Atropos. Superciliary shields many, forming a prominent 

 arched series. Asia. 



b. Subcaudal plates four-rowed at the tip. America. La- 



chesina. 



6. Lachesis. Head ovate ; crown scaly. 



B. Head more or less shielded. 



c. Subcaudal plates tivo-roived ; tail ending in a spine ; cheeks 



not scaly ; head-shields ivith some additional plates in 

 front. Trigonocephalina. 



7. Trigonocephalus. Dorsal scales keeled. 



d. Subcaudal plates one-roived ; tail ending in a spine ; cheeks 



not scaly ; head shielded. America. Cenchrina. 



8. Cenchris. ~ - 



e. Subcaudal plates one-rowed ; tail ending in a rattle ; cheeks 



scaly ; head more or less shielded. America. Crotalina. 



9. Crotalophorus. Head with nine large shields extending 

 behind the eyes. 



10. Uropsophus. Head with scales behind ; temporal scales and 

 labial shield moderate. 



11. Crotalus. Head Avith scales behind ; temporal scales and la- 

 bial shields very small, convex. 



I. Head scaly, tvith small shields on the edge of the forehead and eye- 

 broivs ; tail ending in a spine ; cheeks scaly. 



a. Subcaudal plates two-rowed to the tip (Craspedocephalina). 



1. Craspedocephalus. 



Scales lanceolate, keeled : hinder labial shields largest. Con- 

 fined to the New World. 



