SUBORDER A 



CONDYLARTHRA 



121 



Family 3. Phenacodontidae Cope. 



Upper molars quadrangular, somewhat broader than long, with two external, 

 tioo intermediate, and two internal cusps. First and second premolars simpde in 

 both jaws ; third premolar with from one to two accessory tubercles ; fourth inferior 

 premolar submolariform, but narrow; fourth superior premolar large, with two 

 external and one internal cusps, and two small intermediate tubercles. Lower molars 

 with three cones on the anterior half of the crown and on the talonid. Tibial facet of 

 the astragalus medially grooved; fibula articulating with the astragalus only ; navicular 

 coming in contact laterally with cuboid and calcaneum. Fore and hind feet penta- 

 dactyl, semidigitigrade ? 



Phenacodus primaeviis Cope. Lower Eocene of Wasatch beds, Wyoming. 

 A, Skeleton greatly reduced in size. B, Upper and lower teeth of one side, "/a- (After Cope.) 



The Phenacodontidae are distinguished from the Periptychidae by the 

 complication of the premolars, the higher and more compact tarsus, and 

 the longer neck of the astragalus. Of the two species of the genus Phenacodus 

 represented by skeletons, from the Eocene Wasatch beds of Wyoming, P. 

 primaevus Cope (Fig. 151) attained the size of the wolf, while P. wortmani 

 Cope, from the Wind River beds, was as large as the fox. The dorsal 

 vertebrae are fifteen in number, with from five to six lumbar vertebrae. 



Tetraclaenodon Scott (Protogonia Cope; Euprotogonia Earle) (Fig. 152). 

 Teeth bunodont, very similar to those of Hyracotherium. Extremities 

 slender. Lower Eocene ; Torrejon beds, New Mexico and Montana. 



