VERTEBRATA. 309 
always by reversing 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 
b ; Le 
Fic. 289.—a, Head of a harmless Snake (upper view); }, heads of various venom- 
ous Snakes. 
short, thick tail. In the harmless Snakes, the head grad- 
ually blends with the neck, and is covered with plates; the 
teeth are comparatively numerous in both jaws; the pupil 
is round, and the tail tapering. This rule, however, has 
many exceptions. 
2. Lizards may be likened to Snakes provided with 
four limbs, each having five digits." The body is cover- 
ed with horny scales. All have teeth, which are simple 
in structure; and the halves of the lower jaw are firmly 
united in front, while those of Snakes are loosely tied to- 
gether by ligaments. Nearly all have a breast-bone; and 
the eyes (save in the Gecko) are furnished with movable 
lids. In the common Lizards and Chameleon, the tongue 
is extensile. The tail is usually long, and naturally snaps 
off at the twelfth caudal. The Chameleon has a prehen- 
sile tail. The Iguana is distinguished by a dewlap on the 
