566 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



m.l. 



cy 



^ <^ C^ 



Fig. 280. —Upper teeth of thyaenodontids, right side, showing 

 the grinding surface. A, tSinopa, Wasatch and Bridger. 

 B, ^Triternnodon, Bridger. C, ^PUrodon, upper Eocene and 

 lower Oligocene of Europe. D, ]Hymnodon, White River. 

 The dotted line connects the first molar of each. For 

 comparison is added X, ^Oxycena, one of the tOxyaenidse. 

 C and D are much larger than the others, but all, except 

 X, are reduced to the same length. (After Matthew.) 



and the third 

 molar, though 

 small and not sec- 

 torial, had not 

 been lost ; the 

 two external cusps 

 were connate, but 

 not completely 

 fused together and 

 the posterior ridge 

 was not so well 

 developed as in 

 ^HycEnodon, nor 

 was the fourth 

 upper premolar so 

 nearly a carnas- 

 sial. The lower 

 molars were shear- 

 ing blades, but 

 distinct vestiges 

 of the heel re- 

 mained. So far 

 as they are known, 

 the skull and 

 skeleton resem- 

 bled those of 

 '\Hycenodon. 



]Hycenodon and 

 iPterodon were 

 evidently derived 

 from a group of 

 small fcreodonts 

 which, in the lower 

 and middle Eo- 

 cene, were spread 



