412 UNGULATA 



structure of the skeleton is essentially rhinocerotic, the skull having 

 an ossified nasal septum, and a huge frontal prominence for the 

 support of a very large horn. The teeth are extremely hypsodont, 

 with the enamel plicated to a remarkable degree, and unlike those 

 of Rhinoceros. The genus is evidently a very specialised one. 



The other genera we have to notice are more generalised types. 

 Of these the North American Hyracodon, with the full typical 

 number of teeth, and without nasal horn, appears to connect the 

 Khinoceroses with the Lophiodont Hyrachyus. The genera Amynodon 

 and Metamynodon (Fig. 174), from the American Tertiaries, are 

 forms allied to the Rhinoceroses, with the full number of incisors 

 and canines, and the hinder lobe of the last upper molar not aborted. 

 The loAver canines are either upright, or less proclivous than in the 

 Rhinoceroses ; in Metamynodon the premolars are reduced to 2. 

 Molar teeth from the Phosphorites of Central France, described 



Fio. 174. Right half of the palatal surface of the cranium of Metamynodon planifrons, from 

 the Upper Miocene of North America. (After Scott and Osborn.) 



under the name of Cadiircotherium, are constructed on the general 

 plan of those of the Rhinoceroses, although distinguished by their 

 extreme narrowness ; this type of tooth being very similar to that 

 found in Homalodontotherium from Tertiary deposits in Patagonia. 

 The latter has the full number of teeth, without any diastema in 

 the series. Until we have some knowledge of the skeleton of these 

 remarkable forms nothing definite can be said as to their serial 

 position. 



Families LAMBDOTHERIID.E, CHALICOTHERIID^, AND 

 TITANOTHERIID.E. 



These families contain a large number of more or less nearly 

 related extinct types from Tertiary beds of both the Old and New 

 Worlds, some of which present most remarkable deviations from 

 the ordinary Ungulate structure. All are characterised by their 

 brachydont molars, which depart widely from the normal lophodont 

 type. The upper molars consist of four columns, of which the two 

 external ones are expanded to form an outer wall ; the posterior 

 pair being connected in some cases by an oblique transverse ridge, 



