ORDER UNGULATA. 



1365 



crescentoid ; and in all the forms in which that tooth is known 

 there is no third lobe to the last lower true molar. The height of 

 the cheek-teeth varies considerably, their crowns being tallest in 

 Elas mother iu7n. One or more dermal horns may be attached to 

 the fronto-nasal region ; and when two of these appendages are 

 present they may be either placed one behind another in the middle 

 line, or in a pair on either side of this line. The digits of the pes 

 are apparently always three, but there may be either three or four 

 in the manus. One of the most generalised forms is Hyracodon, 

 from the Lower Miocene of Nebraska, in which the dental formula 



is I. -, C. -, Pm. -, M. -. There were apparently only three digits ; 



the neck and limbs were slender and Horse-like ; and there was no 

 trace of a nasal horn. This genus was in all probability a de- 

 scendant of the Lophiodont Hyrachyus, but does not appear to 

 have been the progenitor of the true Rhinoceroses. In some re- 

 spects still more generalised is the genus Amynodon (Orthocynodon), 

 from the Middle and Upper Eocene of North America, in which 

 the dental formula was the same as in Hyracodon. The lower 

 canines were nearly upright ; there was a short diastema ; the pre- 

 molars were unlike the true molars ; and it is believed that the 

 manus had four digits. Allied to this genus is Metamy?iodo?i, from 

 the Miocene of the United States, in which the skull (fig. 1237) 



has a strong sagittal crest, the premolars are reduced to — , the lower 



2 



canines have become somewhat proclivous, and the upper premolars 

 are much more like the true molars. These two genera are regarded 

 by some of the American 

 palaeontologists as indicat- 

 ing a distinct family — the 

 Amynodontidce. — and are 

 also looked upon as the 

 ancestors of the true Rhi- 

 noceroses. In the Old 

 World there is, however, 

 the genus Cadurcotherium 

 of the Quercy Phosphorites, 

 which may possibly lay 

 claim to this position, al- 

 though it may indicate a 

 lateral branch allied to the 

 Toxodontia. Unfortunate- 

 ly, only detached teeth are at present known, so that the dental 

 formula cannot be determined. The upper true molars (fig. 1238) 



1238. — A left upper true molar of Cadtcr- 

 cotherium cayluxense ; from the Upper Eocene 

 of France. 



