HISTORY OF THE ARTIODACTYLA 381 



and many similar problems can find no definitive solution. 

 The question of relationship with other families is bound up 

 with that of the origin of the joreodonts ; many characters 

 point to a connection with the fanthracotheres and, from 

 the standpoint of present knowledge, that appears to be the 

 most probable affinity ; but, on the other hand, there are 

 structural features which suggest relationship with the primi- 

 tive camels. Between these and other alternatives, only the 

 recovery of the middle and lower Eocene forms can finally 

 decide. 



Reviewing the long history of the oreodont family from the 

 evolutionary point of view, we find a course of development 

 which differs in several respects from that exemplified by most 

 of the families previously considered : 



(1) There was a general increase in size, though it was far 

 from steady, and almost every genus had larger and smaller 

 species, and in some of the phyla the species were far larger than 

 in others. The members of the '\Leptauchenia phylum were 

 very small and no member of the family ever attained to more 

 than moderate size. 



(2) The upper molars early lost the fifth cusp, and after 

 that there was little change in the dentition, except that in 

 the \Merychyus and \Leptauchenia phyla the grinding teeth 

 became hypsodont. 



(3) There was great variety in the modifications of the 

 skull, each phylum having its own peculiarities. The orbit, 

 which was open behind in the Uinta fProtoreodon, was closed 

 in the White River and all succeeding genera. In the \Mery- 

 cochoerus series, the skull first enlarged, with little change in 

 proportions, then elongated the facial region, then shortened 

 the face and so reduced the nasals as to indicate the presence of 

 a proboscis, culminating in the grotesque, ape-like skull of 

 ^Pronomotherium. In the ]Leptauchenia phylum the skull 

 became depressed and flattened and the face was invaded by 

 great openings, or vacuities; the tympanic bullae were enor- 



