SUBORDER C 



PERISSODACTYLA 



153 



with pits. Orbits closed posteriorly. Lateral toes shorter than the third. 

 Middle Miocene ; Deep Kiver 



beds, Montana. P. " Desmatippus" 

 crenidens Scott. " Jnchippus " 

 brevidens Marsh. P. texanus 

 Leidy. Miocene ; Texas. P. 

 nebrascensis Peterson. Miocene ; 

 Nebraska. Common in the Lower 

 Miocene. 



Archaeohippus Gidley. Middle 

 Miocene. 



Altippiis Douglass. Cheek 

 teeth high, second upper pre- 

 molar elongated. Metapodials 

 A. tazus Douglass. 



Fig. 208. 



Parahippus nebrascensis Peterson. Miocene, Nebraska. 



Skull and lower jaw. V4- (After Peterson.) 



very long. Middle Miocene ; Montana. 



Subfamily 4. Equinae. 



Orbits completely surrounded by bone. Cheek teeth hypsodont, with nearly 

 always copious cement. First premolar very small, iveak, often absent ; other j^re- 

 molars molariform. Superior molars with elongated crescentic intermediate tubercles, 

 which form closed pits. First inner cusp large, developed as basal pillar, second 

 small. Inferior premolars and molars with large internal pillar developed as a 

 double loop at the point of union of each crescent. Incisors chisel-shaped, the enamel 

 on the crown infolded and forming a pit. Radius and ulna anchylosed together, 

 the latter very slender at the distal end. Manus and pes tridactyl or monodactyl, 

 lateral metapodials rudimentary, either with short phalanges or reduced distally to 

 tapering splint bones. 



Fossil in the Pliocene and Pleistocene of Europe, Asia, northern Africa 

 and North America, occurring also in the Middle and Upper Miocene of the 

 latter continent. In South America only in the Pleistocene, and at present 

 distributed over the whole earth, but indigenous only in Europe, Asia and 

 northern Africa. 



Merychippus Leidy (Fig. 209). Milk teeth brachyodont with little 

 cement ; premolars and molars moderately hypsodont with abundant 

 cement. Enamel of the upper teeth more or less infolded. Middle Miocene 

 to Lower Pliocene ; North America. M. insignis Leidy. M. sejunctus Cope sp. 



Protohippus Leidy (Figs. 210, 211, D). Protocone of the upper molars 

 joined to the protoconule. Folding of the enamel very slight. Cement 

 copious. Inferior molars as in Equus, but lower. Without lachrymal fossa. 

 Manus and pes tridactyl, the lateral metacarpals and metatarsals very slender, 

 the toes not reaching the ground, as in Hipparion. Low Pliocene (Loup 

 Fork beds) ; North America. P. perditus, placidus, mirabilis Leidy. Pliocene, 

 P. cumminsi Cope. 



Pliohippus Marsh. A doubtful genus. Malar and lachrymal fossa lai'ge. 

 Teeth very similar to those of Protohippus. Lateral metapodials sometimes 

 without phalanges. Lower Pliocene (Loup Fork beds) ; North America. 

 P. pernix, P. robustus Marsh. 



Hipparion Christol {Hippotherium Kaup) (Figs. 211, ^; 212, 213). Incisors 



