ARSINOITHERIUM. 63 



presence of horns in the two suborders does not seem to be a character of much 

 importance, for they differ both in situation and character and, moreover, are found 

 in many other groups. On the other hand, the skulls differ widely in the form 

 of the occipital surface, the arrangement of the external auditory meatus, the form of 

 the premaxillse 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 important 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 are 

 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 oi 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. 



