320 COMPARATIVE ZOOLOGY. 



scales, the others have bony plates combined with 



1. Ophidia, or Snakes, are characterized by the absence 

 of visible limbs; 164 by the great number of vertebrae, 

 amounting to over four hundred in the great Serpents; 

 by a corresponding number of ribs, but no sternum ; and 

 110 true eyelids, the eyes being covered with a transparent 



FIG. 298. Adder, or Viper (Pelias berus). England. 



skin. The tongue differs from that of nearly all other 

 Reptiles in being bifid and extensile. The mouth is very 

 dilatable. The skin is frequently shed, and always by re- 

 versing it. Snakes make their way on land or in water 

 with equal facility. 



As a rule, the venomous Snakes, as Vipers and Rattle- 

 snakes, are distinguished by a triangular head covered with 

 small scales ; a constriction behind the head; two or more 

 fangs, and few teeth; small eyes, with vertical pupil; and 

 short, thick tail. In the harmless Snakes, the head gradu- 

 ally blends with the neck, and is covered with plates ; the 

 teeth are comparatively numerous in both jaws ; the pu- 



