TRANSACTIONS OF WAGNER 

 20 



WHITE RIVER SELENODONTS 



characterized by the same great prominence of the external buttresses and 

 ribs, while in the mandible they are broader and lower. 



The skull is very remarkable, not only for its bizarre appearance, but 

 also for its extreme modernization in certain respects, and for the great dif- 

 ference between the males and females. This skull retains the tylopodan 

 form only in a very modified way, this form being shown in the prominent 

 sagittal and occipital crests and in the long, slender, and tapering muzzle. 

 The cranium is much shortened, the orbit being shifted almost entirely behind 

 the teeth, and the face is bent down upon the basi-cranial axis, as in the 

 Cavicornia, a feature which has not been 'found in any other White River 

 mammal. The auditory bulla is hollow and very small. The zygomatic 

 arch is short, but quite heavy, and the orbit is completely enclosed by 

 bone. The nasals are extremely short, making the anterior narial opening 

 exceedingly large, much as in the Saiga antelope. The premaxillaries are 

 edentulous and their alveolar portion is thin and depressed, but the spines 

 are broad and the incisive foramina very narrow. The mandible is quite 

 pecoran in appearance, with a long, slender, horizontal ramus, and a rather 

 low and broad ascending ramus, with the angle hardly projecting at all 

 behind the condyle. This latter feature is in marked contrast to most ot 

 the tylopodan genera. A resemblance to the oreodonts is found in the shape 

 of the coronoid process, which is very low and tapers abruptly to a blunt 

 point. 



The sexual differences are very striking. In the male skull we find a 

 pair of compressed, club-shaped, and somewhat horn-like processes on the 

 parietals, short spines on the frontals, while the free margins of the maxil- 

 laries rise into massive and everted bony plates. In the females these vari- 

 ous protuberances are very feebly indicated. 



In the vertebral column the neck is moderately elongate and the vertebrae 

 are not of the characteristically tylopodan structure, but the axis is elongate, 

 with a broad, flattened odontoid process, like that of Poebrotherium, and a 

 great hatchet-shaped neural spine, like that of Leptomeryx. 



The scapula is, in general, like that of Poebrotherium, but is narrower 

 and has a veiy much smaller prescapular fossa, a somewhat shorter acromion, 

 and a less prominent coracoid. The ulna and radius are separate, except 

 that in old individuals they may unite distally. The head of the radius has 

 a broad intercondylar convexity, somewhat as in the oreodonts ; distally it 

 has no contact with the pyramidal. In the carpus the trapezoid and mag- 



