3-20 
COMPARATIVE ZOOLOGY. 
scales, the others have bony plates combined with 
scales. 
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 
no true eyelids, the eyes being covered with a transparent 
Fig. 298— Adder, or Viper (Pelias berm). 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- 
