1906.] 



ARTICULATION OF THE VERTEBRATE JAW. 



119 



tlie lips do the work allotted to the incisors of most other 

 herbivora, the angle is far nearer a right angle. The skull of the 

 Dugong (in which horny plates take the place of incisors) seems 

 to attempt another solution of the difficulty. Another angle is 

 introduced into the jaw, bringing the anterior third of the jaws 

 into a line parallel with the ascending ramus (text-fig. 42). A 

 less marked tendency to introduce the second angle may be seen 

 in some other animals : — among the Pigs, in 6'i(>s longlrostris ; 

 among the Ruminants, in the Ohevrotain Tragulas javanicus 

 (text-tig. 43). 



Text-fig. 42. 



Diagram of jaws. IVpe 3. 



Showing the introduction of a second angle to procure wide separation of the 

 incisors with slight separation of the molars. 



Text-fi,o-. 43. 



Skull of Tragzdus Javanicus. 



Having briefly studied the principle in the Mammals, I next 

 turned to the Reptiles. Most of these have jaws of type 1 : the 

 lower jaw articulates with the skull in the plane of the teeth. 

 The Snake, however, has a mouth in which (owing to the great 

 mobility of the quadrate) the jaw-principles of type 1 and type 2 

 are combined in a very remarkable manner. 



