ARSINOITIIERIUM. 
g:] 
presence of horns in the two suborders does not seem to be a character of mucli 
importance, for they differ both in situation and character and, moreover, are found 
in many other groups. On the otlier hand, the skulls differ widely in the form 
of the occi])ital surface, the arrangement of the external auditory meatus, the form of 
the premaxilloe and of the anterior palatine foramina. The mandible is very unlike 
in the two groups, particularly in the form of the angular region. 
In the dentition of the Amblypoda the following im])ortant differences may be 
noticed : — (1) the upper incisors are small and weak and the canines large and 
tusk-like; (2) there is a diastema behind the canine in both jaws; (3) the cheek- 
teeth are relatively small and are brachyodont ; (4) in wear the upper teeth tend to 
give V-shaped surfaces. In a former paper (Geol. Mag. [5] vol. i. 1904, p. 158) the 
present writer suggested that possibly the hypsodont molars of Arsinoitherium might 
have been derived from some form of Amblypod tooth, but further consideration has 
shown that this is almost impossible. Moreover, the great differences above referred 
to show that the two groups are very widely separated. 
Comparison with the Hyracoidea is very difficult on account of the small size of 
the modern representatives of the group and our complete ignorance of the 
skeleton of the large extinct forms. In the early Hyracoidea there is a tendency for 
the anterior teeth (incisors, canine, and premolars) to form a continuous series, the 
anterior pair of incisors being more or less enlarged and separated from one another 
in the middle line by a short interval, as in Arsinoitherium. The molars arc 
brachyodont, but their pattern is such that if there were an increase in height 
accompanied by an increasing infolding of the outer wall, they might give rise to a 
type of tooth very similar to the molars of Arsinoitherium, as was explained above. 
Since, however, these early brachyodont Hyraces are contemporaries of Arsinoitherium, 
it is not suggested that they are closely related ; but the circumstance that they 
both seem to have originated in the same region lends some probability to the 
belief that they may have had a common ancestry in the late Secondary or earliest 
Tertiary period. 
Dr. Ameghino has suggested in a letter that Arsinoitherium may be related to 
the Toxodontia, but in spite of some superficial resemblances such a relationship 
does not seem at all probable. We may mention only two of the many important 
differences between the two types : (1) the structure of the molars seems to be 
totally dissimilar, and it is very difficult to see how the two types can be connected 
with one another; (2) the structure of the tarsus is very different in the two cases, 
that of the Toxodontia apparently showing the cuboid in articulation exclusively with 
the calcaneum. 
It is to be hoped that remains of earlier members of the Barypoda may be found in 
the Lower Eocene beds of Egypt, and that by these the question of the affinities 
of the group may be definitely settled. 
