UNGULATA. 



323 



and a more slender animal, Hyracotherium (with the formation 

 of the transverse ridges on the molars more or less incom- 

 plete), which is generally claimed as an ancestral horse. In 

 other genera the two ridges on the upper molars are connected 

 by a longitudinal crest which forms an outer wall ; and here 

 among other types discovered in the Eocene of America may 

 be placed Hyrachyus, which is regarded very plausibly as an 

 ancestral rhinoceros. One of the best-known skeletons is that 

 commonly described under the name of 



Hyracotherium (figs. 183, 184). The skull in this genus is much 

 elongated, with slender snout, and the long nasal bones projecting forwards 



FIG. 183. 



llyracotlierium tapirinum ; right upper (A) and lower (B) molars and premolars, 

 four-thirds nat. size. L. Eocene (Wasatch Formation) ; Wyoming. de. t 

 deuterocone ; de d ., deuteroconid ; hl d ., hypoconulid ; me*., metaconid ; pr., 

 protocone ; pr 4 . , protoconid ; tr., tritocone. (After Wortman.) 



above the narial opening. The sagittal crest of the cranial region is pro- 

 minent, and there is a large postorbital process of the frontal ; but the 

 orbit is not completely separated by bone from the temporal fossa. A 

 small infraorbital foramen is distinct. The slender mandible is much 

 elevated in the coronoid region, and the rami are fused at the symphysis 

 in the adult. The dentition is complete, comprising the usual 44 teeth ; 

 but there are three short diastemata in the upper jaw (behind i. 3, c, and 

 pm. 1), and two in the lower jaw (behind c and pm. 1). The incisors are 

 relatively small, in regular series, and the canines are scarcely enlarged. 

 The characters of the premolar-molar series are well shown in fig. 183. 

 The odontoid process of the axis vertebra is known to be a simple peg ; 

 and the centra of the cervical vertebrae are slightly opisthocoelous. The 



212 



