230 TROPICAL MEDICINE AND HYGIENE 



absence of a mental groove (fig. 94, m.g.). Furthermore, 

 the squamosal bone is rudimentary, and the pterygoid 

 does not nearly reach to the quadrate. 



(B) The two families to which the poisonous snakes, 

 or Thanatophidia (Greek Odvaros = death, and ofas = a 

 snake) belong are the Colubridce and the Viperidce. In 

 both of them squamosal and transpalatines are present 

 (fig. 95), the pterygoid extends backwards to the quadrate 

 (fig. 95), and the mandible is without a coronoid bone. 

 In both of them certain aquatic Colubrines, both 

 venomous and non-venomous, excepted the ventral 

 shields are so much enlarged transversely that their ends 

 can usually be seen in a lateral view. Or, as Major Wall 

 puts it, if the snake is looked at as it lies on a table, 

 belly upwards, the most that can be seen besides the 

 ventral shields is a glimpse of a single line of lateral 

 scales. Of course, it is assumed that the belly of the 

 snake is not distended with food ; if the stomach be 

 full this character is fallacious. 



Though a typical viper is easy to distinguish from a 

 typical Colubrine snake, there are not a few vipers that 

 may be mistaken for Coin brines, and vice versa. The 

 safest criterion is the form of the maxilla : in Colubridce 

 the maxilla is horizontal, and is usually of some length ; 

 in Viperidce the maxilla is a very short bone, is not 

 horizontal, and can be erected for action quite per- 

 pendicular to the palate. 



COLUBRID.E. 



(i) Colubridce. In the great majority of snakes of this 

 family the head is covered with large symmetrically 

 arranged shields (figs. 92 to 94), and the ventral shields 

 are so much broadened transversely except in certain 

 aquatic species that their ends are visible in a lateral 

 view : the maxilla is horizontal, and in many species 

 carries a considerable number of teeth. The family is a 

 very large one, numbering nearly 1,300 species, or more 

 than three times as many species as all the other families 



