320 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



as to the origin of the selenodont molars of the Artio- 

 dactyla ; and my own as to the origin of a similar 

 structure in the molars of certain multituberculate Pro- 

 totheria. In the former the mastication is ental ; in 

 the latter it is proal, as shown by Osborn. 



In the accompanying figure from Ryder the move- 

 ments of the lower jaw in mastication of lophodonts, 

 are diagrammatically represented, a represents the 

 movement in Carnivora, and in the orthal bunodonts, 

 as the pigs, b shows a slight lateral movement be- 

 lieved by Ryder to exist in the wart hog (Phacochoe- 



Fig. 88. — Diagram of excursion of lower jaw in mastication; from Ryder ; 

 a-b, orthal ; c-f, ental. 



rus). c represents the movement in kangaroos, pha- 

 langers, and tapirs. In </ a theoretical intermediate 

 movement is represented, such as Ryder supposed to 

 have characterized the Anchitherium. In e the usual 

 movement among ruminants is depicted, as is seen in 

 the deer, etc. In / the wider excursion of the jaw is 

 that seen in the giraffe, camel, and ox. In these move- 

 ments from b to f, the lower jaw is moved transversely 

 across the upper jaw from one side to the other. 

 Some of the Diplarthra masticate on one side of the 

 jaw when performing this movement, and some on the 

 other. That is, in passing the lower jaw across the 

 face of the upper, some masticate the food on the side 



