68 COMPARATIVE ZOOLOGY. 



they are planted also in the roof of the month. 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. 33. Poison Apparatus of the Rattle- fangs ordinarily lie flat upon 



snake: or, gland, with duct, leading to ,1 , , , , 



the fangV/; m, elevator muscles of the the gums, bl.lt are brought 



jaw, which in contracting, compress the j t vertical position in 



gland ; , 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 into 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- 



