AKSlNOITilEIilUM. 
17 
l^ehind and at a slightly higher level there is in the cast another prominence (aud.), 
which probably marks the position of the exit of the auditory and perhaps also of 
the facial nerve. 'I'here is no condylar foramen, and consequently the cast shows 
no trace of the hypoglossal nerve. No clearly defined sella turcica is present, as 
the whole of the surface of the basicranial axis is flat or slightly convex from side 
to side. 
I'he central portion of the cerebellum {ch.) seems to have been small and 
flattened : the limit between it and the cerebral hemispheres is marked by a slightly 
developed tentorial ridge. Laterally the cranial cavity is prolonged into a pair of 
large fossae, which may have been wholly or in part occupied by the lateral lobes 
of the cerebellum {l.l.). 
This brain is larger in proportion to the bulk of the animal than in the 
Amblypoda, and seems to be fairly well developed for an Eocene Ungulate. At the 
same time it should be noted that in the earlier and contemporary genus Mceritherium 
the brain is relatively very much larger (see below). 
Upper Permanent Dentition. — The teeth of Arsinoitherium (Pis. I.-V. ; text-fig. 6) 
form perhaps the most extraordinary dentition found among the Ungulates, and the 
inteipretation of the peculiar structure of the molars which is here given must be 
regarded as provisional until it is either confirmed or destroyed by the discovery of 
earlier members of the group, in which the modification of the teeth from more 
ordinary types is less extreme. 
The modification of the individual teeth has not been accompanied by any 
reduction in their number, the dental formula being i, 3, c. |, pm. 4, m. 3, and, except 
for a short interval between the median incisors in the upper jaw, the teeth form a 
closed series and wear to a common level throughout. The most striking character- 
istics of the dentition as a whole are the extreme hypsodonty of the teeth (at least 
for an Eocene mammal) and the great difference between the molar and premolar 
teeth (text-fig. 6). 
The upper molars (PL V. figs. 6-8; text-fig. 6, A) are remarkable for the great height 
of their crowns and for the considerable changes of pattern their grinding-surfaces 
undergo in the course of wear. Each tooth consists essentially of two high transverse 
crests or columns {p.c. and a.c.), which posteriorly are slightly convex from above 
downwards and concave from side to side, while anteriorly they are convex in the 
latter direction. The posterior column is not situated immediately behind the anterior 
column, but a little towards the inner side, so that its outer edge is nearly opposite 
the middle of the anterior column (PI. V. fig. 6). Externally the two columns are 
completely separated by a deep vertical cleft, but internally they are only divided for a 
short distance in the upper part of the unworn crown, being united beneath this 
by a strong crest which seems to belong to the cingulum (cl). This cingulum is 
D 
