PLiOLOPnus. 651 



the fourth premolar tooth, instead of one. Most of the dentition and a 

 good deal of the skeleton of the English P. vulpicejas have been described by 

 Owen, and I have described nearly as much of the P. vintanus from New- 

 Mexico. From these descriptions it is evident that the characters are in 

 general those of Hi/racoiheriiim. Professor Ow^ens's diagnosis of Pliohphus 

 does not specify the generic distinction above mentioned, for the reason 

 that the mandibular dentition of Hijracotherium was unknown at the time he 

 wrote. He did, however, give his reasons for distinguishing the genus from 

 the latter, and on this account the generic name must be retained. I for- 

 merly adopted for this genus a name introduced by Professor Marsh, but 

 without certainty that he really had the present form in view in proposing it. 

 Professor Marsh mentions the similarity between the fourth premolar and the 

 first true molar. I regarded this statement as a definition of the genus, and 

 therefore adopted it. An examination of the type specimen of Pliolophus 

 vulpiceps in the British ^Vfuseum permitted me by Professor Owen, has satis- 

 fied me that the American species I have called Oroilieriuni must be placed 

 in the same genus. 



Species. — Besides the P. vidpiceps from the English Suessonian, four 

 species have been described from North American formations, and a sixth 

 is now added. Several of them are only known from mandibular rami, 

 which are, in two of them, fragmentar}^. The Lophiotherium sylvaticum 

 Leidy probably belongs to this genus, if not to the one to which Dr. Leidy 

 referred it, a point which cannot be positively settled without a more com- 

 plete specimen. I refer it to the immediate neighborhood of the P. cinctus 

 as the most probably correct course. The species differ as follows: 



Internal tubercle of third interior premolar smaller than the external anterior, 

 and posterior in position. 



/? Anterior inner tubercles of molars bifid. 

 Dei>th of ramus at M ii, .017 P. vintanus. 



Pii Anterior inner tubercle simple. 



Depth of ramus at Pm. iv, .0155; length of Pm. ii, iii and iv, .018 P. cristonensis. 



Deptli of ramus at Pm. iv, .0135; length of Pm. ii, iii and iv, .0185 (Owen). 



P. vulpiceps. 

 Depth of ramus at Pm. iv, .012; length of Pm. ii, iii and iv, .015 P. lavi. 



aa Internal tubercle of third inferior premolar as large as the external, and not 

 much posterior to it. 



