156 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



with small scales ; a constriction behind the head ; two 

 or more fangs, and few teeth ; small eyes, with vertical 



FIG. 134. Adder, or viper ( Vipera berus}. England. 



pupil; and short, thick tail. In the harmless snakes, 

 the head gradually blends with the neck, and is cov- 

 ered with plates ;. the teeth are comparatively numerous 



FIG. 135. a, Head of a harmless snake (upper view"); b, heads of various venomous 



snakes. 



in both jaws ; the pupil is round, and the tail tapering. 

 This rule, however, has many exceptions. 



