324 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



from tubercle-bearing (bunodont) ancestors. This de- 

 scent has witnessed an increased depth of the infold- 

 ing of the crown, as represented in the accompanying 

 figure. Now, in the table of masticatory types above 

 given it is shown that in the bunodont type of the 

 Suoidea the mastication is orthal, and a gradual in- 

 crease in the length of the lateral excursions of the 

 lower jaw has been shown to have resulted in the ental 

 mastication. Thus has structure kept pace with func- 

 tion in the evolution of the selenodont dentition. 



Fig. 93. Chirox plicatus Cope, palate and molar teeth from below, three- 

 halves natural size. From Puerco bed of New Mexico. From American Nat- 

 uralist, 1887, p. 566. 



The general structure of the dentition in the Proto- 

 theria Multituberculata is similar to that of the Glires. 

 The incisors in the Plagiaulacidae, Chirogidae, and Poly- 

 mastodontidae have structure and functions generally 

 similar to those of that order. The result in the 

 form and function of the molar dentition has been sim- 

 ilar to that observed in the Glires. The postglenoid 

 process is probably absent in these animals ; in any 

 case the mandibular condyle is rounded, and is not 

 transverse. Prof. H. F. Osborn has pointed out to 

 me that mastication was performed by a fore-and-aft 



