the Genera of Oreodontidce. 289 



Upper Oligocene- the oreodonts lack the first digit of the 

 manus and are termed Eporeodon which otherwise re- 

 sembles especially Eucrotaphus having a large bulla. It 

 still retains the brachydont dentition, though the forms 

 have increased in size and it is to this genus that all the 

 John Day forms are assigned. 



The Lower Miocene produced a wealth of oreodonts, 

 both as to numbers and as to variety of adaptations. The 

 least modified genus is Mesoreodon which is distinguished 

 from Eporeodon by the teeth having begun to show an in- 

 crease in height, moderate hypsodonty and medium type 

 of limbs being the distinguishing features of this genus. 

 I can find no characters to distinguish the genus Mery- 

 coides (Douglass) from Mesoreodon, the dentition being 

 perhaps less hypsodont, but however, already showing a 

 beginning of the elongation of the teeth. The genus Mes- 

 oreodon then is typically a Lower Miocene one, only one 

 species, M. longiceps, being found in the Middle Miocene, 

 and this species might equally well be placed in the genus 

 Ticholeptus for it has a small antorbital fenester, and 

 seems to represent the transition from Mesoreodon to 

 Ticholeptus. The line of less specialized oreodonts is 

 continued into the Middle Miocene by the genus Ticholep- 

 tus represented by at least four species. This genus is 

 distinguished by the increased hypsodonty of the molar 

 teeth and by the large fenester in front of the orbit. It 

 may turn out that the type species, T. zygomaticus, should 

 be placed among the species of Merychyus as it has a slen- 

 der narrow skull, but the point can not be settled until the 

 feet are found. At any rate there is a group of four or 

 fiYQ Middle Miocene oreodonts with the medium type of 

 feet, medium type of elongation of the teeth and large 

 fenestra in front of the orbits and this term is needed to 

 designate a very distinct stage in the evolution of the 

 group. 



The genus Metoreodon so far as is known, that is the 

 skull and jaws, seems also to belong to this group and ex- 

 tends the line into the Pliocene, but it is not a direct 

 descendant of Ticholeptus for it has no antorbital fenes- 

 ter. For the same reason and because it is of more mas- 

 sive build I am confident it was not derived from Mery- 

 chyus. It has the most advanced dentition of the ore- 

 odonts being strongly hypsodont and is distinct in that the 

 premolars have an infolding of the enamel on the external 

 side. To me the genus seems to be derived from Mesore- 



