TRANSACTIONS OF WAGNER 

 26 



UINTA SELENODONTS 



are strongly convex externally, with a strong cingulum between them, while 

 the inner ones are thick and but little compressed. M^- has a very large fifth 

 lobe which quite equals the outer crescents in height and length ; on its inner 

 side is a small separate cusp. 



As a whole, the dentition of Protylopus is closely similar to that of Poe- 

 brotherium, and just what we should expect to find in a genus ancestral to 

 the latter. The incisors and canines are very much alike in the two genera, 

 but in the White River form the diastema between p 1 and p- in both jaws is 

 increased in correspondence with the elongation of the face, and the other 

 premolars have become much elongated antero-posteriorly and have enlarged 

 their accessory cusps ; the molars display a decided tendency to assume the 

 hypsodont structure, and on those of the upper jaw the external buttresses are 

 reduced in size. It is of interest to observe that in Poebrotkcrium the upper 

 milk-premolars, especially dp A , are more like the true molars of Protylopus 

 than are the true molars of the White River genus. 



II. The Skull (Plate II., fig. 5) is very like that of Pocbrotherium, though 

 with many obvious differences. In the first place, it is very much smaller than 

 in the smallest species of the latter; secondly, the face, and especially its 

 anterior portion, is much less elongated, while the cranium is narrower and 

 less capacious. The orbit is smaller and much more widely open behind, and 

 the auditory bulla has not attained such an exaggerated size. On the other 

 hand, there is the same short sagittal crest, the same broad, lozenge-shaped 

 forehead, narrow, tapering face, and slender jaws. Protylopus has already 

 acquired the tylopodan physiognomy in a very marked degree, and the skull 

 immediately recalls that of the llama to the observer, though, of course, the 

 peculiarities are much less accentuated than in the later members of the 

 series. So closely does the present genus approximate the contemporary 

 members of certain other artiodactyl series, that, in the absence of the inter- 

 mediate forms, it would be difficult to establish its connection with the 

 modern Tylopoda. 



The skull is, in some respects, of quite an advanced type of structure. 

 The cranium is short and, for an animal of so early a date, capacious and well 

 rounded; it supports a short and inconspicuous sagittal crest; the occiput is 

 low, broad at the base, narrow and rounded at the top, and its crest likewise 

 is weakly developed. The face is rather long, the orbit being shifted far back, 

 so that its anterior margin is placed above m^. While remaining of nearly 

 uniform vertical height throughout, the face narrows much anteriorly, but 



