202 NATURAL HISTORY. 
with a transparent membrane which is shed with the 
skin. The tongue is soft, and forked at its end, and it is 
not very sensitive. The smell and the touch are both 
rather dull. Serpents have teeth, but not for mastication. 
They only serve to retain their food. 
335. The species of this order may be grouped in two 
classes—the Viperine and the Colubrine Snakes. The 
Viperine Serpents have a peculiar venomous apparatus. 
There are two teeth or fangs in the upper jaw, connect- 
ed with the gland in which the poison is made. They 
are movable, and when the animal does not wish to use 
them, they lie backward, concealed along the roof of the 
mouth. When the serpent bites, he throws these fangs 
forward, and, at the same time, a muscle, pressing on the 
gland, forces out the venom, which passes along a canal 
in the fang. Most of the Colubrine Snakes are ‘not ven- 
omous, and those which are have stationary instead of 
movable fangs. 
336. There are two families of the Viperine Snakes— 
the Viperidz and the Crotalidz. The Viperide belong 
exclusively to the eastern hemisphere. Those of the trop- 
ical regions are the most venomous. To this family be- 
longs the Horned Viper, so called from a small pointed 
horn above each eye. This is supposed to be the Asp, 
from whose bite Cleopatra died. The Puff Adders of 
Africa also belong to this family. 
337. Of the family Crotalide, the true Rattlesnakes, 
Fig. 164 (p. 203), are confined to this country, but there 
are other species found in Asia. The rattle consists of 
a number of thin, horny appendages, which are loosely 
jointed together, and which make a rustling noise when 
the snake moves. The number of joints is increased, up 
to a certain amount at least, with each casting of the skin. 
338. The Colubrine Snakes have two families—the Co- 
lubride or Colubers, and the Boidz or Boas. The fami- 
ly of Colubers contains more than half the whole number 
of species of Snakes. Of the comparatively few of these 
