256 BIOLOGICAL LECTURES. 



suggestive. The upper premolars are like those of Protoreodon, 

 and, as in the latter genus, the upper molars have an unpaired 

 cusp, but the most cursory glance at the pattern of these 

 molars is sufficient to recognize their essential likeness to 

 those of Agriockcerus. The tarsus is also strongly suggestive 

 of the ancestral position here assigned to the genus, but it is 

 provoking that the ungual phalanges are all missing. Whether 

 or not the ancestral position rightfully belongs to Protagrio- 

 chcerus, it is certain that this genus and Protoreodon bring the 

 two families very close together and make it altogether prob- 

 able that both groups lead back to a common ancestor in 

 Bridger times. 



It may well prove to be the case that some of the relations 

 between Uinta and White River genera here suggested will be 

 shown by future discoveries to be erroneous. It matters little, 

 however, whether we have chosen precisely the proper ances- 

 tors for the later forms ; there still remains the highly impor- 

 tant and significant fact that in this Uinta fauna all these 

 different families are seen to be obviously converging back to 

 some common term, and that they are much nearer together 

 than they afterwards became in White River times. It is this 

 fact which justifies us in maintaining the essential unity of all 

 the indigenous American selenodonts, diversified and highly, 

 specialized as many of them eventually became. Having the 

 great continent practically to themselves, they adopted many 

 rdles, which naturally resulted in a greater or less likeness to 

 the forms among the ancient Pecora and Tragulina which were 

 playing similar parts in the eastern hemisphere. This explains 

 the tantalizing and elusive likenesses to European genera, 

 which have so long misled us, and shows why it was impos- 

 sible to make any satisfactory arrangement of these American 

 genera in European families or even suborders. 



When we attempt to trace these various lines back of the 

 Uinta, we find ourselves very much in the dark, because of our 

 ignorance of the more ancient forms. The little Bridger genus 

 Homacodon comes very near to filling the requirements of the 

 common ancestor of all these groups, and it is exceedingly 

 probable that the family of which it is the representative will 



