794 THE WHITE EIVER FAUNA. 



The species are distinguished as follows : 



I. Anterior triangle of inferior molars with the two inner cusps subequal and deeply 

 separated. 



a. The anterior triangle not much more elevated than the posterior. 



Larger ; last two molars 4.5™"° long ; ramus 4°"° deep at middle P. fugax. 



Smaller; last two molars 4.5"'"; ramus 3""° P. tricuspid. 



Smallest ; last two molars 2.7"" ; ramus 2"" P. huntii. 



aa. The anterior triangle twice as much or more elevated than the posterior. 



Larger ; last two molars 4.5°"" P. scalare. 



II. The anterior cusp of the elevated anterior triangle insignificant, the posterior inner 

 cusp much larger. 



A groove extending to the base of the anterior cusp on the inner side ; heels of molars 



supporting two cones P- inarginale. 



No groove separating anterior cusps; heels of molars very low P. alternans. 



Peratherium fugax Cope. 



Herpetotherium fugax Cope. Paleontologic.al Bulletin No. 15, August, 1873, p. 1. Annual Report U. S. 



Geol. Surv. Terrs., 1873 (1874), p. 467. 

 Plate LXII; figs. 1-9. 



This species is represented by the anterior half of the cranium with 

 mandibular rami, with nearly complete dentition, of one individual ; and 

 by larger and smaller parts of mandibles of fourteen others. 



The portion of cranium preserved is somewhat crushed by pressure, 

 but it was evidently wider in proportion to its length at the frontal region 

 than in the existing Didelphidce of America. The muzzle is broken away, 

 but the inferior incisor teeth show that it was not elongate. The skull begins 

 to expand in front of the foramen infraorUtale, and at the lachrymal region 

 overhangs, by a little, the maxillary border. The orbits do not appear to have 

 been large. The malar bone, if it existed, is lost from the specimen. The 

 nasal bones are narrowed by the elevated maxillaries for the distal two thirds 

 of their length. They then rather abruptly expand to double the width, and 

 terminate in a wide slightly convex posterior border. Either the maxillo- 

 premaxillary suture is situated well in advance of the canine tooth, or else 

 it has become obliterated by coossification. The foramen infraorbitale exterius 

 is above the third premolar tooth. 



The superior canine is compressed, but neither the anterior nor posterior 

 edges are acute. The first premolar is smaller than the others, and its crown 

 is rather obtuse. It is separated by a short interspace from the second. 

 The latter has a trace of a posterior basal tubercle ; its edges are not acute. 



