HYPOHIPPUS. 



203 



GENUS HYPOHIPPUS Leidy 1S58. 

 Proc. Acad, Nat. Sci. Phila. 1858, p. 2G. 



Synonym. — Drymohippus Merriam, subgenus H. (Drymohippus) nevadensis, 1913. 



Genotype. — Hypohippus (Anchitherium) affinis Leidy, from the Niobrara River, near Fort Niobrara, Nebraska, 

 Lower Pliocene, Nebraska formation, Procamelus-Hipparion zone. 



This genus, which was originally based on a single superior deciduous premolar, has become securely established since 

 1858 through the discovery of a series of important specific stages, namely, Hypohippus equinus Scott from the Middle 

 Miocene, Ticholeptus-Merychippus zone, of the upper sections of the Deep River formation of Montana, osborni 

 Gidley of the typical Middle Miocene of Pawnee Creek, Colorado, H. nevadensis Merriam from Stewart Valley, Nevada, 

 probably Upper Miocene, and finally H. matihewi Barbour from the Lower Pliocene of Devil's Gulch, Nebraska. 



These horses are believed to have been persistently browsing and forest-living, with habits somewhat like those of 

 the tapirs and the primitive species of browsing rhinoceroses. Unlike all other late Tertiary equines the feet are isotri- 

 dactyl, the lateral digits continuing to rest on the ground and functioning. As browsers hypsodontism is slowly acquired 

 although it is incipient in the Lower Pliocene Hypohippus matthcwi, which is subhypsodont. In the grinders none of the 

 secondary folds appear, such as the crochet, and the hypostyle remains small and primitive, not passing beyond the stage 

 observed in Mesohippus. 



The characters common to the six species known, including the Hypohippus zitteli of the Pliocene of China, are the 

 following: 



1. Protoloph and metaloph distinct and continuous, cones and conules lophoid, pro to- and metaconules not ex- 

 panded. 



2. Hypostyle a looped or rounded eingule, not free from the postcingulum ; internal and external cingula vestigial 

 or atrophied except in //. matthcwi, which has strong internal cingulum. 



3. Posterior grinding teeth, m|, reduced. 



4. Inferior molars with external cingulum. 



5. Skull relatively small, microcephalic. 



G. Preorbital or facial region relatively short. 



7. Lachrymal fossa deep, narrow, and superior in position (H. osborni). 

 S. No malar fossa (H. osborni). 

 9. Nasals expanded anteriorly. 



10. Limbs relatively short, feet isotridactyl. 



11. Mts. Ill articulates laterally with both cuboid and mesocuneiform. 



TICHOLEPTUS-MERYCHIPPUS ZONE. MIDDLE MIOCENE. 



Hypohippus equinus Scott 1893. 

 Text Fig. 167. 



Anchitherium equinum sp. nov., Scott, W. B. "The Mammals of the Deep River Beds," Amer. Naturalist, Vol. XXVII, No. 319, 

 July, 1893, p. 661, no figure. "The Mammalia of the Deep River Beds," Trans. Amer. Philos. Soc. N. S. Vol. XVIII, 1894, p. 94, 

 PI. iii, figs. 25-28. 



Horizon and locality. — Upper beds of Deep River (Lower Loup Fork), Mont. Middle Miocene, Deep River Forma- 

 tion, Deep River Valley, Mont. Type collected by I. Benet. 



Type. — Princeton Mus. 10404 (Scott, Trans. 1894, p. 95) "The type specimen of the species consists of a frag- 

 mentary skull (with the dentition almost complete), several vertebrae from different regions, the fore limb (lacking the 



