2 54 



TROPICAL MEDICINE AND HYGIENE 



{A semilunar supra-nasal shield (fig. 104, 7z) Bitis. 

 (Africa and Arabia.) 

 No supra-nasal shield Pseudocerastes. 

 (Persia and Baluchistan.) 



{Scales on the sides of the body with sharp keels ; 

 tail from about one-sixth to about one-fourth 

 the total length Atheris. 

 (Africa.) 

 Scales on the sides of the body with serrated 

 keels (fig. 102, a d] ; tail short 8 



{Ventral shields of the ordinary curved form Echis. 

 (Asia and Africa.) 

 Ventral shields with a keel on either side (fig. 103) Cerastes. 

 (Africa, Arabia and Syria.) 



Enlarged 



FIG. 102. Lateral scales, a, b, Echis serrated keel, surface and profile ; 

 c, d, Cerastes serrated keel, surface and profile ; e, /, Bitis -sharp keel, 

 surface and profile. 



Nai Size 

 FIG. 103. Ventral shield of Cerastes, showing lateral ridges. 



(b] SYNOPSIS OF GENERA OF CROTALINVE. 



Tail ending in a rattle (Rattlesnakes) i 



Tail not ending in a rattle (ordinary loreal-pit- 



vipers) 2 



Head with large symmetrical shields. Small, 



slender snakes Sistrurus. 



(N. America.) 



Head with small scales alone, or with small 

 scales and some small shields. Large bloated 



snakes as a rule Crotalus. 



(N. and S. America.) 



