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 
Rhinoceroses 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 lower canines are either upright, or less proclivous than in the 
Rhinoceroses ; in A/etamynodon the premolars are reduced to 3. 
Molar teeth from the Phosphorites of Central France, described 
i Ss ee WYN ‘ 
Fic. 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 Cadurcotherium, 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&. 
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, 
