334 MAMMALIA. 



Hyracodon. The parietal region of the skull in this, as in the other 

 primitive rhinoceroses, is raised into a sharp sagittal crest ; the freely 

 projecting nasal bones are slender and smooth. The complete series of 44 

 teeth is present, and the premolars resemble the molars, while the canines 

 are uniform with the incisors ; but there is a short diastema in both jaws 

 between the premolars and the canine. The molars do not exhibit the 

 "crochet" and "crista." The skeleton denotes a slightly-built animal, 

 with longer limbs and a longer neck than are characteristic of the true 

 rhinoceroses. Both feet are definitely proved to have been tridactyl. 

 The genus occurs in the Lower Miocene (White River Formation) of 

 Nebraska and Colorado, and the typical species, Hyracodon nebrascensis, 

 has a skull about - 25 m. in length. Some molar teeth from an early Tertiary 

 Formation in Siebenbiirgen, Himgary, have also been referred to this 

 genus. 



FIG. 192. 



Aceratherium (Aphelops) megalodus; skull and mandible, one-sixth nat. size. 

 U. Miocene (Loup Fork Formation) ; Colorado. (After Cope.) 



Aceratherium (fig. 192). The earliest remains apparently of rhino- 

 ceroses found in Europe (detached teeth and limb-bones from the Upper 

 Eocene or perhaps Lower Miocene Phosphorites of Quercy, France) are too 

 fragmentary for discussion ; but the Miocene and Pliocene remains are more 

 satisfactory, and a hornless rhinoceros, Aceratherium, with a rudimentary 

 fourth toe (no. v) on the fore foot can be clearly recognized. The sagittal 

 crest of the skull is not acute, but always more or less flattened, and the 

 nasal bones are small, freely projecting. The premaxillary region is 

 slender and elongated. The dental formula of the more typical species is 



