AESINOITHEEITJM. 17 



Behind 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. There is no condylar foramen, and consequently the cast shovs's 

 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. 



The central portion of the cerebellum (cb.) 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 Moeritherium 

 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 

 interpretation 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. §, c. -^, pm. i, m. g, 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 (cJ). This cingulum is 



