320 
COMPARATIVE ZOOLOGY. 
two have horny scales, the others have bony plates com¬ 
bined with scales. 
L Ojphidia , or Snakes, are characterized by the absence 
of visible limbs; 164 by the great number of vertebrge, 
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. 29S.—Adder, or Viper (Pelias herns)* 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- 
