HYPOHIPPUS. 



207 



MERYCHIPPUS PANIENSIS-M. SEJUNCTUS ZONE. 10. EARLY MIDDLE MIOCENE. 



This zone is typified by the Pawnee Creek, northeastern Colorado. 



Hypohippus osborni Gidley 1907. 

 Plates 5.9, 6.4, 35.2,4, 39.8,11, 51.2,5. Text Figs. 51, 168. 



Hypohippus osborni sp. nov., Gidley, James W. "Revision of the Miocene and Pliocene Equidse of North America," Bull. Amer. 

 Mus. Nat. Hist., Vol. XXIII, Art. XXXV, Nov. 26, 1907, pp. 930, 931, no figure. 



Horizon and locality. — Middle Miocene, Pawnee Creek formation, Merychippus and Ticholeptus zone, Pawnee beds, 

 northeastern Colorado. Type collected by Barnum Brown, Amer. Mus. Expedition of 1901. 



Type. — Amer. Mus. 9407. A skull and nearly complete skeleton. Measurements: (Gidley) p L m 3 .151; molar 

 series, m 1-3 , .069; total length of skull .371. Paratype. Amer. Mus. 9395, a palate with complete dentition, associated 

 with lower jaws and other parts of the skeleton. 



Type figure— Plates 5.9, 6.4, 35.4, 39.8, 11, 51.2,5, text Figs. 51, 168 of this Memoir. 



Characters. — (Gidley, 1907) " As indicated by the teeth, this species is intermediate in size and progressive 

 development between H. affinis, the type of the genus, and IT. equinus (Scott), the latter being the smallest and least 

 specialized. Compared with II. equinus the principal differences are, (1) cheek teeth relatively longer crowned; (2) 

 outer walls of upper cheek teeth more deeply curved; (3) protocone and hypocone more compressed anteroposteriorly; 

 and (4) the incisors are relatively wider .... The unusually complete skeleton on which the genus Neohipparion (N. whitneyi) 

 was founded affords an opportunity for an interesting comparison of two wholly different types of horses. In general 

 proportions H. osborni is longer necked, longer bodied and shorter limbed than N. whitneyi. The relatively smaller head 

 with its brachyodont uncemented teeth, its less vertical depth, its more anteriorly placed orbits which are not inclosed 

 behind, presents a totally different appearance from that of N. whitneyi which is very much nearer the typical modem 

 horse. A comparison of the feet also shows some marked differences. In II. osborni the lateral toes are long and slender, 

 the lateral ones being so greatly reduced as to render the foot practically monodactyl." 



A referred specimen is Amer. Mus. 9395. 



PROCAMELUS-HIPPARION ZONE. LOWER PLIOCENE. 



Hypohippus affinis Leidy 1858. 

 Plate 35.1. Text Fig. 169. 



Anchitherium {Hypohippus) affinis, Leidy, Joseph. "Notice of Remains of Extinct Vertebrata, from the Valley of the Niobrara 

 River, collected during the Exploring Expedition of 1857, in Nebraska, under the command of Lieut. G. K. Warren, U. S. Top. Eng., 

 by Dr. F. V. Hayden, Geologist to the Expedition," Proc. Acad. Nat. Sci. Phila. Vol. X, 1858, p. 26, no figure. Hypohippus 

 affinis, "The Extinct Mammalian Fauna of Dakota and Nebraska," Jour. Acad. Nat. Sci. Phila. (2) Vol. VII, 1869, p. 311, PI. xxi, 

 figs. 11, 12. 



Horizon and locality. — Lower Pliocene, " Nebraska formation," Hipparion and Procamelus zone, Niobrara River, 

 near Ft. Niobrara, Nebraska. Name of collector not given. 



Type. — U. S. National Mus. 573 (cast, Amer. Mus. 10771). Fourth upper milk premolar, dp 4 , of left side. Meas- 

 urements: (Gidley, 1906) Nat. Mus. 573, dp 4 a. p. .027, tr. .029. Another specimen is referred by Gidley, Amer. Mus. 

 10834, also found in the Lower Pliocene, Nebraska formation at Big Spring Canon. Measurements: p3 a.p. .028, tr. .020; 

 p 4 , .0275, tr. . 021, height of crown, inside, .020, outside .025; m -0285, tr. .020. 



Type figure. — Text Fig. 169 of this Memoir. 



