DAKOTA AND NEBRASKA. 87 



form an acute isosceles triangle between the anterior angular processes of the frontals. 

 The triangle varies in length, and degree of acuteness. The sides are more or less 

 slightly convex, especially towards the base. In a few instances the apex is observed 

 to be more or less rounded, and rarely notched. 



The auditory bulla of Oreodon Culhertsoni is remarkable for its small size, in com- 

 parison with that of recent ruminants and siiilline animals, and with that also of the 

 other species of the genus. It forms a moderate, convex prominence about half the 

 size of the contiguous post-glenoid tubercle. It extends in a ridge to the paramastoid 

 process, and in a vaginal ridge and auditory process to form the inferior boundary of 

 the external auditory meatus. Between it and the basi-occipital and basi-sphenoidal 

 there exists a wide crescentoid interval, at the bottom of which the periotic bone is 

 visible. 



Among the better-preserved specimens of skulls, and fragments of others, of 

 Oreodon Cidbertsoni, the following are especially mentioned as exhibiting the 

 characters and variations in the species : 



1. A nearly complete skull with the lower jaw and almost all the teeth, repre- 

 sented in figure 1, plate VI. This specimen, viewed as a whole, represents very well 

 the ordinary variety and size of the skull of Oreodon Cidhertsoni. From its some- 

 what narrower character, lesser robustness, and smaller canine teeth, it is suspected 

 to have belonged to the female. 



In the specimen, the sagittal crest begins to widen unusually early, as the temporal 

 ridges gradually diverge from it near the middle of its course, enclosing upon the 

 foi'e part of the parietal a long narrow triangle. The anterior border of the parietal 

 is deeply notched to receive the angular summit of the frontals. The flxce is 

 comparatively narrow, deep, and prominently convex above. The orbital entrance 

 is vertically oval. The lachrymal fossa, in advance, is broad, deep, and encroaches 

 on the contiguous part of the maxillary bone. The nasal bones are narrow, 

 nearly parallel at the sides but slightly wider forward. Behind, they are slightly 

 convex; in front more convex. The posterior nasal triangle, formed by the back 

 extremities together of the nasal bones is subacute, about eleven lines long in 

 the middle, and seven and a half lines wide between the pointed ends of the anterior 

 angular processes of the frontals. 



2. A skull less complete than the former, with part of the lower jaw. In form, 

 size and most of the details it closely resembles the former. The sagittal crest is 

 longer and remains undivided until it reaches the fronto-parietal suture. The orbits 

 are rotund. The lachrymal fossa3 are narrower and deeper. The fore part of the face 

 is narrower. 



3. A mutilated skull without the lower jaw. It more nearly resembles the latter 



