666 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



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Fio. 280. —Upper teeth of fhycenodontids, right side, showing 

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

 B, t Tritemnodon, Bridger. C, ^Pterodon, upper Eocene and 

 lower Oligooeue of Europe. D, i Hycenodon, White River. 

 The dotted line connects the first molar of each. For 

 comparison is added X, ]OxytBna, One of the fOxysenidse. 

 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 

 \Hyoenodon. 



^Hycenodon and 

 IPterodon were 

 evidently derived 

 from a group of 

 small fcreodonts 

 which, in the lower 

 and middle Eo- 

 cene, were spread 



