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27 



UINTA SELENODONTS 



does so gradually, not displaying the sudden constriction at p A which is so 

 characteristic of the modern members of the group. 



Unfortunately, the base of the skull is so crushed and concealed in 

 matrix that very little can be made out concerning its structure. The exoc- 

 cipitals are broad and low, forming a very broad projection above the foramen 

 magnum, with a shallow fossa on each side of the prominence ; the paroccip- 

 ital processes are quite long and are compressed antero-posteriorly, tapering 

 to a blunt point. The supraoccipital is rather narrow and its posterior face 

 is made up entirely of the median prominence, which extends upward from 

 the exoccipitals, the lateral fossa? being cut off by the narrowing of the 

 occiput. The supraoccipital appears to extend over upon the dorsal side 

 of the cranium, but as the sutures in this region are mostly obliterated, it is 

 not possible satisfactorily to make out the limits of the bone. The mastoid 

 would seem not to be exposed upon the surface of the skull. 



The parietals are relatively very large and roof in nearly the whole of 

 the cerebral fossa ; the sagittal crest is short, low, and thin, soon bifurcating 

 into two long, low, and curved temporal ridges, which terminate at the frontal 

 suture near the postorbital processes. Anteriorly the parietals are deeply and 

 broadly notched to receive a median prolongation of the frontals. 



The tympanic is not very well preserved in any of the specimens, but 

 enough remains to show certain important differences from that of Poebrotlie- 

 rium. For ease of comparison, I shall quote the account of this bone in the 

 latter genus, which I have given elsewhere ('gib, p. 15) : " The tympanies are 

 inflated into enormous bullae, which in both species of Poebrotherium are 

 relatively much larger than in the recent genera and are more rounded. In 

 the small species, P. Wilsoni, they are larger and less compressed than in 

 P. labiatum, and in both the long diameter is directed nearly parallel to 



the cranial axis As in the Tylopoda generally, the bulla is filled with 



cancellous bony tissue. The external auditory meatus is a closed ring, open- 

 ing slightly upward and backward; its rim does not project at all beyond 

 the surrounding parts of the squamosal." To this description it may be 

 added that the bulla abuts against the paroccipital process, with which it 

 unites suturally for the greater part of its length, extending also consider- 

 ably in advance of the postglenoid process. The auditory meatus is not a 

 tube, but a mere opening, with a raised lip, into the bulla. In all these 

 respects the bulla of Protylopus is different from that of the White River 

 genus, the first and most obvious difference being its very much smaller 



