RHINOCEROS OF NEBRASKA. 557 
and in the specimen in which the forehead is preserved, an inch behind their com- 
mencement, they are within five lines 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 Rhinoceros Indicus, and is 
relatively narrow, but bulges in the middle so that its extent of surface is really 
not reduced. 
_ The foramen magnum occipitis is subrotund, about ten lines in diameter. Its 
margin above is notched, and overhangs considerably posterior to the inferior 
margin, 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 maxilla—The body of the lower jaw is vertically convex, and its base 
more convex antero-posteriorly than in Acerotherium incisivum. The ramus is 
much depressed internally, and is thin. The posterior maxillary foramen is situated 
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 down 
nearly to the edge of the alveoli. The formula for the molars is :. 
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. 3 
The last molar exhibits a tendency to the formation of a posterior valley by the 
presence 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 bounded 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 
