TRANSACTIONS OF WAGNER 

 J 08 



UINTA SELENODONTS 



In each instance the Uinta genus has a higher and narrower tarsus than its 

 presumable White River successor. 



Certain difficulties which oppose the phylogenetic arrangement here 

 sketched, deriving Agriochcerus from Protagriochasrus, suggest themselves, 

 though they seem to be of no very great weight. (1) The difference between 

 the two genera is apparently greater than that between Oreodon and Protoreo- 

 don, or that between Poebrotherium and Protylopus, and therefore it might seem 

 that the structural gap is too great for the time interval. As a matter of fact, 

 however, we know that development has proceeded at different rates in different 

 phyla, and there is no known reason why change in the agriochcerids should 

 not have been rather more rapid than in the oreodonts. (2) The lack of 

 diastemata in the dentition of Protagriochcerus and their presence in the White 

 River genus might by some be regarded as an insuperable objection to the 

 derivation of the latter from the former. As shown in a previous chapter, this 

 is an untenable assumption ; Protylopus has no diastemata, in Poebrotherium 

 they are very short, but in Gompliotlierium and all the subsequent genera of the 

 camels they are long. What has happened in the camel series may equally 

 well have happened among the agriochcerids. (3) It would naturally be ex- 

 pected that the very remarkable foot-structure, and especially the ungual 

 phalanges of Agriochcerus, should be more distinctly foreshadowed in its Uinta 

 ancestor. So far as the tarsus of Protagriochcerus is concerned, it is just what 

 it should be in the ancestral form, and until the ungual phalanges have been 

 recovered we shall not know how great the difference between the two genera 

 in this respect really is. 



The available evidence thus goes to show that Agrioehoerus is the descend- 

 ant of Protagriochcerus , or of some very similar type, and that for all practical 

 purposes the latter may serve to represent the actual ancestor. If this con- 

 clusion be valid, then certain interesting and somewhat unexpected corollaries 

 will follow from it. (1) The oreodonts and agriochcerids are very closely 

 related, and the marked differences displayed by the White River and later 

 representatives of the two families were due to a rapid divergent evolution, 

 especially on the part of the Agriochcrridce. In the latter family the skull 

 retained most of its primitive features, such as the elongate cranium, open 

 orbits, convex lachrymals, etc., though with an elongation of the muzzle and 

 consequent production of moderate diastemata in the dentition. The teeth 

 underwent a curious modification, while the limbs and especially the feet 

 assumed a most extraordinary character, quite unique among the artiodactyls, 



