the Genera of Oreodontidce. 



291 



being very like Mesoreodon. As known today Merychyus 

 ranges from the Lower to the Upper Miocene. 



In Phenacoccelus, Leptauchenia, Cyclopidius, and 

 Pithecistes we have a group of highly specialized oreo- 

 donts in which the sknll has a pair of frontal fenes- 

 tra as well as the antorbital fenestra. Unfortunately 

 except for Phenacoccelus the limbs have been very 

 scantily described bnt where known they seem to be 

 of moderate length and not stocky. The group is a pro- 

 gressive one, being more specialized in the order named 

 above and very peculiar in that the nasals become fused 

 to the premaxilke bones. Phenacoccelus is the least 

 specialized and has the frontal and antorbital fenestra 

 separate, also moderately long and slender limbs. In 

 Leptauchenia the skull is short and wide and the frontal 

 and antorbital fenestra have become confluent, but the 

 dentition is retained in full. Cyclopedius has the same 

 general characters, but the incisors are reduced to f ; while 

 the little known Pichecists has gone a step further and 

 has but one incisor (the third) still remaining in the lower 

 jaw. 



„ u. 



.,M 



M 



„. U- 



L 



n. 



U 





Locene 



OliQOcene 



Olijocene 



Oligocene 



Miocene 



riiocehe 



Miocene 



PI locetie 





/ 



oW* oS \ 



> :5?< 



P ^ s 



_--TVono 



nothcvion 



1 



--ProboYeodot 



\ 



X 



^ 



\ 



\ 



5>^ 





Meforeodon 





^9*k 







v- 



•\_ych_yus- 





















w'hehacocoek 

















V 

















\ 



^ 















% 



/ 















%■ 



Cyclopidius 























Fig. 4. — Diagram of the Evolution of the Oreodon. 



Am. Jour. Sci.— Fourth Series, Vol. L, No. 298.— October, 1920. 

 21 



