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UINTA SELENODONTS ° 



oreodonts and agriochcerids, it seems highly probable that all the Uinta 

 selenodonts are closely related to one another, and there is no reason appar- 

 ent to doubt that they were all derived from the same family, possibly even 

 from the same genus, of Bridger times. The only important differences in 

 the dentition between Leptoreodon and Protylopus are as follows : In the for- 

 mer the upper canine is large and the lower canine has assumed the shape 

 and function of an incisor, while p T is caniniform and takes its place. Con- 

 siderable diastemata separate the first from the second premolar in both jaws. 

 Except in very minor details, the other premolars and the molars are alike in 

 the two genera. We also find great similarity in the character of the skull, 

 although in Leptoreodon the face is longer and of greater vertical height, and 

 the mandible has in both the same great extension behind the condyle. In 

 both the feet and limbs are very much the same, save that in Protylopus the 

 metapodials are more elongate and the lateral digits, especially those of the 

 pes, far more reduced. Homacodou may serve as well for the Bridger ances- 

 tor of one genus as of the other. 



Looking forward to White River times, the descendants of Leptoreodon 

 cannot be so distinctly identified, though it seems highly probable that one of 

 these is Protoceras. Unfortunately, there is a wide, unbridged gap in this line 

 between the Uinta and the upper White River, and unless the problematical 

 Stibarus should belong in this series we have yet to find the successive steps 

 that led up to Protoceras. The latter genus has in the structure of its skull a 

 number of deceptive resemblances to the higher Pecora, resemblances which 

 the rest of the skeleton does not sustain. Highly significant is the fact that 

 in the males the upper canine is a curved tusk, abraded upon the posterior 

 face by the caniniform p T . Except that they have increased in size, the limbs 

 and feet show surprisingly little advance over those of Leptoreodon ; almost 

 the only changes to be noted are the reduction of the fibula and of the lateral 

 digits of the pes, with the partial anchylosis of the ulna and radius. Compar- 

 ing Protoceras with Leptoreodon, one is surprised to find that, in view of the 

 long time interval which separates them, they should differ so little. All the 

 facts, as we at present know them, point to the conclusion that Protoceras was 

 derived from Leptoreodon or from some very similar genus. 



In my former paper upon the osteology of Protoceras I called attention 

 to the many points of resemblance between this genus on the one hand and 

 Leptomeryx and Hypertragnlus on the other : " This family represents a group 

 of White River selenodonts, each of whose genera has become more or less 



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