68 
COMPARATIVE ZOOLOGY. 
they are planted also in the roof of the mouth. With 
few exceptions, they are conical and curved (Fig. 33). In 
the Serpents they are longest and sharpest; and the ven¬ 
omous species have two or more fangs in the upper jaw. 
These fangs contain a canal, 
through which the poison 
is forced by muscles which 
compress the gland. The 
bones to which they are at¬ 
tached are movable, and the 
Fig. 38.— Poison Apparatus of the Rattle- fangS Ordinarily lie flat UpOll 
snake: q, eland, with duct, leading to ,1 , , , 
the fang,/; m, elevator muscles of the the gUlUS, blit are brought 
jaw, which in contracting, compress the j n {- 0 a vertical position in 
gland ; s , salivary glands on the edge of r 
the jaws; n, nostril. the act of striking. As a 
rule, the teeth of Reptiles are simply soldered to the bone 
which supports them, or lodged in a groove; but those of 
Crocodiles are set in sockets. Reptilian teeth are made 
of dentine and a thin layer of cement, to which is added 
in most Saurians a coat of enamel on the crown. 
In the majority of Mammals, the teeth are limited in 
number and definite in their forms. The number ranges 
from 1 in the Narwhal (but the longest tooth in the king¬ 
dom) to 220 in the Dolphin. The average is 32, occur¬ 
ring in Ruminants, Apes, and Man; but 44 (as in the 
Hog and Mole) is called the typical or normal number, 
and this number is exceeded only in the lower groups. 
When very numerous, the teeth are of the Reptilian type, 
small, pointed, and of nearly equal size, as in the Porpoise. 
In the higher Mammals, the teeth are comparatively few, 
and differ so much in size, shape, and use, that they can 
be classed linto incisors, canines, premolars, and molars. 
Such a dental series exhibits a double purpose, prehension 
and mastication. 'The chisel-shaped front teeth are fitted 
for cutting the food, and hence called incisors. These 
vary in number : the Lion has six in each jaw; the Squir 
