UNGULATA. 337 



have also carried a minute horn. The front teeth are completely absent, 

 and the two anterior premolars have also disappeared. The remaining 

 teeth (pm. 3, 4, m. 1 3) are deepened (hypsodont) and prismatic in shape, 

 like those of a horse ; the valleys between the ridges are completely filled 

 with cement; and the ridges themselves are elaborately crimped. The 

 known limb-bones are quite rhinocerotic in type. The only known 

 species, E. sibiricum, has a skull at least a metre in length. 



The true horses, or Equidae in the most restricted sense of 

 the term, appear first in the Upper Miocene of North America 

 and in the Lower Pliocene of Europe. They are all agile 

 animals with one large functional digit on each foot, and a pair 

 of lateral digits, which are sometimes complete though small, 

 sometimes mere splint-like rudiments. The incisors are chisel- 

 shaped, with the apex folded into a longitudinal pit ; the 

 canines are diminutive ; and the grinding teeth are deepened 

 (hypsodont), the valleys always filled with the cement, and 

 the two ridges (homologous with those of Palaeotheriidse, etc.) 

 connected by a cross-crest. The premolars 2 4 resemble the 

 molars, and the foremost premolar is rudimentary or absent. 

 The orbit is completely surrounded by bone. 



FIG. 194. 



Protohippus sejunctus ; palatal aspect of skull, one-third nat. size. U. Miocene 

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



Protohippus (figs. 182, 194). In the early American genus, Proto- 

 hippus or Merychippus, from the Upper Miocene (Loup Fork Series) of 

 Nebraska and Colorado, the grinding teeth of the adult resemble those of 

 a generalized Equus, only they are somewhat less hypsodont; but the 

 milk-molars are curiously similar to those of Anchitherium. The small 

 lateral digits are complete, but the ulna and fibula are already imperfect. 



w. 22 



