the Genera of Oreodontidtf. 287 



* teeth moderately hypsodont — Mesoroedon. 

 ** teeth strongly hypsodont, antorbital fenes- 

 ter — Ticholeptus. 

 *** teeth strongly hypsodont, no antorbital 

 fenester, but groove on outside of premo- 

 lars — Metoreodon. 

 2 — limbs short and stocky. 



a — skull elongate and heavy, teeth brachodont — 



Promerycochoerus. 

 b — skull short, nasals shortened, teeth hypsodont 



— Merychochoerus. 

 c — skull short, nasals very short, teeth hypsodont — 

 Pronomotherium. 

 3 — limbs long and slender, small antorbital fenester — 



Merychyus. 

 4 — Limbs moderate length, both frontal and antorbital 

 fenestra ; 



a — frontal fenestra small — Phenacocoelus. 

 b — frontal and antorbital fenestra confluent — 



Leptauchenia. 

 c— as above, but incisors ? — Pithecistes. 

 d — as above but incisors £ — Cyclopidius. 



From their origin in the Upper Eocene the Oreodontise 

 have stood out clearly from among the other, selenodont 

 artiodactyls, and though they survived to the latter part 

 of the Pliocene were less adaptive than other artiodactyls, 

 at the same time retaining through their history many of 

 the primitive artiodactyl characters. The Uinta forms 

 show that the group came off from that division of artio- 

 dactyls which retained the anterior intermediate cusp 

 (protoconule), but to just which one of the later groups it 

 is most nearly related is not and can not be determined 

 definitely until more complete material from the Eocene 

 is available. The group is peculiar in that its history is 

 purely American, none of the forms participating in any 

 of the migrations by which American types found their 

 way to other continents. 



The Uinta genera Protoreodon and Protagriochoertis 

 are very close to each other and would hardly be separated 

 were it not that some of the species of that period clearly 

 are more closely related to the Oligocene generus Ore- 

 odon, others to Agriochoerus. These genera then repre- 

 sent the beginning of the divergence of the two closely 

 related families. Both genera have the anterior interme- 

 diate cusp on the upper molars, both have the orbit open 



