59 



HYRACHYUS. 



An extinct genus of odd-toed pachyderms, under the above name, was 

 originally inferred from specimens of fossils obtained during Professor Ha\ r - 

 den's exploration in Wyoming, in 1870. One of the specimens, represented 

 in Fig. 11, Plate II, consists of the greater portion of a ramus of the lower 

 jaw, without teeth, found on Smith's Fork of Green River. The other speci- 

 men, represented in Fig 12, consists of a lower-jaw fragment, with several 

 teeth, of a young animal, from Black's Fork of Green River. 



Hyrachyus is closely related with the extinct tapiroid genus Lophiodon, the 

 remains of which belong to the early Tertiary formation of Europe. In a less 

 degree, also, it is related with the rhinoceros-like Hyracodon of the Mauvaises 

 Terres of White River, Dakota. Among living animals, it is most nearly 

 allied to the tapir, and more remotely with the rhinoceros. 



The dental series of the true Lophiodon, if the L. isselense of Issel, 

 France, be viewed as the type of the genus, or of Tapirotherium, as it had 

 been previously named by De Blainville, consists of three incisors, a canine, 

 three premolars, and three molars. The living tapir at maturity has one pre- 

 molar more to the upper series. 



In one species of Hyrachyus at maturity there are- four premolars to the 

 series above and below, as in Hyracodon. Apparently, in a second species 

 there are four premolars in the upper series, and three in the lower, as in the 

 tapir. 



The last lower molar of Lophiodon has a trilobate crown. In Hyrachyus, 

 as in the tapir, it has a bilobed crown. 



The crowns of the lower molars are intermediate in character with those 

 of Lophiodon and Hyracodon. 



The upper molars of Hyrachyus closely resemble those of Lophiodon. In 

 both genera the upper back two premolars have a single lobe to the inner 

 part of the crown representing the inner pair of lobes of the crowns of the 

 succeeding molars in a connate condition. In Lophiodon a ridge proceeds 

 from the inner lobe of the crown of the premolars mentioned to the antero- 

 external lobe. In Hyrachyus, in the corresponding teeth, a pair of ridges 

 proceed from the inner lobe of the crown to both the outer lobes. 



The lower jaw of Hyrachyus has nearly the form and construction of that 

 of the tapir. 



