1884.] 559 [Cope. 



Pithecistes heterodon Cope. 



C'yclopidius heterodon Cope, Proceeds. American Philos. Society, 1877, 

 p. 22. 



In this species the fourth premolar has the same form as in P. decedens, 

 but the first true molar differs much in the more prismatic shape, and the 

 absence of the external vertical ribs. It is quite possible that it does not 

 belong to this genus. 



Ticholeptus beds of Deep river, Montana. J. C. Isaac. 



K.liMH IMFKl S Leidy. 



Proceedings Academy, Philadelphia, 1850, p. 121. Extinct Mammals 

 Dakota and Nebraska, 1869, p. 131 (as family Agrioclmridoe). 



Orbit not closed behind. Fourth superior premolar with two external 

 Vs. Fourth inferior premolar like true molars. Otic bulla inflated. Pre- 

 maxillary bones distinct ; no vacuities in the facial bones. 



This genus commences cotemporaneously with the genus Oreodon, and 

 persists longer, viz. : to the close of the John Day epoch. It represents a 

 distinct line of succession from that which we have been considering, and 

 one which contains but two known terms. Next to Agriochcerus comes, in 

 this line, the genus Coloreodon Cope, which outlasted its predecessor so far 

 as is yet known. It commenced with it in the John Day epoch, and con- 

 tinuing into the North Fork beds, which are of later age, did not ap- 

 pear later. This series Leidy regarded as a family distinct from the Oreo- 

 dontidee. For the present I prefer the view of Gill, that it constitutes a 

 subfamily, the Agriocheerinse. 



This genus presents us with one of the very few cases in the suborder 

 Artiodactyla, in which the last premolar approaches (above) or accom- 

 plishes (below) identity of structure with the true molars. This degree of 

 complication was attained at the same period by both the equine and 

 rhinocerontic lines of Perissodactyla, and all existing members of that 

 order exhibit it. In the Agriochcerhlse it made a beginning, but soon dis- 

 appeared from the earth, and no Artiodactyle has developed such perma- 

 nent premolars successfully since. 



In the characters of the skull this genus is less robust than the Oreodon- 

 tidse ; but the general skeleton remains unknown. 



Five species have been described which are referable to this genus, and 

 two others are now added. One of the former is without premaxillary or su- 

 perior incisor teeth, and I therefore regarded it as representing a distinct 

 genus under the name of Merycopater. It, however, appears that no speci- 

 mens exist in our museums which exhibit this part of the skull in other 

 species of the genus, so it is absolutely uncertain whether Agriochcerus 

 possesses those teeth or not. The species may then be distinguished as 

 follows : 



I. Otic bullae compressed, base anteroposteriorly ovoid. 

 «. Foramen infraorbitale above junction of P-m. iii and iv. 



