R II I X C E 11 S OF NEBRASKA. 557 



and ill the specimen in wliicli the forehead is preserved, an inch behind their com- 

 mencement, they are within five Unes of each other, so that they no doubt joined 

 to form a parietal crest, the superior boundary of the temporal fossa?. 



The occipital surface is much more trilateral than in Ehinoceros Indicus, and is 

 relatively narrow, but bulges in the middle so that its extent of surface is really 

 not reduced. 



Tlie foramen magnum occipitis is subrotund, about ten lines in diameter. Its 

 margin above is notched, and overhangs considerably posterior to the inferior 

 mariiin, so that the foramen is directed more downwards than in R. Indicus. The 

 occipital condyles are more vertical than in the latter. 



The mastoid process forms the posterior boundary of an inverted U-shaped passage 

 to the tympanum, and projects inferiorly nearly as much as the post-glenoid tubercle. 

 The latter relatively to that of Rhinoceros Indicus, is very short, being only ten 

 lines. It is, however, broad and robust and truncated at its extremity. The glenoid 

 surface upon the root of the zygoma is directed rather more outwards and back- 

 wards than in R. Indicus, but upon the post-glenoid tubercle is depressed and 

 directed forward with a slight inclination outwards. 



The post-glenoid tubercle is relatively to that of Rhinoceros Indicus very short, 

 but is broad and robust, and truncated at its extremity. 



Inferior maxilld. — The body of the lower jaw is vertically convex, and its base 

 more convex antero-posteriorly tlian in Acerotherium incisivum. The ramus is 

 much depressed internally, and is thin. The posterior maxillary foramen is situiited 

 about an inch posterior to the last tooth. 



Dentition. — The permanent teeth remain until a late period of life without any 

 reduction of their number, except the normal first of the lower jaw, as illustrated 

 in the skull containing on both sides all the molars, seven in number, worn doAvn 

 nearly to the edge of the alveoli. The formula for the molars is I. 



The upper teeth (Tab. xii. A, fig. 6), bear very great resemblance to those of 

 Acerotherium incisivum. They all possess a basal cingulum, well developed all 

 round, when not obliterated by pressure where the teeth are in contact with one 

 another, except at the inner side of the bases of the internal lobes of the posterior 

 three molars. 



The last molar exhibits a tendency to the formation of a posterior valley by the 

 ])resence of a deep vertical depression at its posterior angle. The bottom of the 

 principal valley is nearly level, and nearly as deep as the crown ; and at its entrance 

 is Ijounded by a mammillary eminence, a constituent portion of the basal ridge. 

 The internal lobes are simple ; the anterior is slightly expanded at its middle pos- 

 teriorly. 



The preceding two molars have their inner lobes directed in a slightly sigmoid 

 course, inwards and backwards. These lobes are simple, expanded at base and 

 internally, and rapidly taper towards their summit. The anterior lobes are dilated 

 at their middle posteriorly, increasing in this disposition successively from the last 

 to the fifth molar. The anterior valley of the fifth and sixth molars is broad, and 

 decreases in depth from the entrance outwards, so that in the attrition to which 



