1906.] 



ARTICULATION OF THE VERTEBRATE JAW. 



119 



the 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 incisoi-s) 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 aiagle may be seen 

 in some other animals: — among the Pigs, in Sus longirostris; 

 among the Ruminants, in the Ohevrotain Tragulus javanicus 

 (text-tig. 43). 



Text-fig. 42. 



Diagram of jaws. Tj'pe 2. 



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

 incisors with slight separation of the molars. 



Text-fig. 43. 



Skull of Tragulus 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. 



