136 



osborn: oligocene, miocene, pliocene eqtjid^e. 



Neotypc characters. — (Gidley, 1906, 1907) Based on fragment of skull, Amer. Mus. 10830. (1) Protocones directed 

 well backward, as in P. perditus; (2) enamel borders of fossettes relatively simple; (3) upper molariform teeth of smaller 

 size than in P. perditus, i. c. diameters of crown; (4) of greater comparative length than P. perditus; (5) of less curvature 

 than P. perditus; (6) fossettes in true molars of less transverse width. (7) A slight depression in the malar bone immedi- 

 ately in front of the orbit, otherwise the facial region full and smooth as in P. perditus; (8) the lachrymal fossa shallow 

 and not sharply defined. 



This species obviously rests upon the redefinition by Gidley through the neotype. Gidley observes (Matthew & 

 Gidley, 1906, p. 141, Gidley, 1907, p. 837) that in the comparative length and relatively slight curvature of the tooth 

 crowns and the general form of the fossettes the grinding teeth resemble Hipparion (Neohipparion) rather than Protohippus 

 yet the protocone and hypocone structure are related to Protohippus. Measurements of neotype, upper jaw Amer. Mus. 

 10830: total length molar-premolar series, p 2 -m\ 102; p 4 a.p. .015, tr. .0183; m 1 a.p. .0132, tr. .017. 



The animal is relatively abundant in the Nebraska formation and several well preserved specimens from Big Spring 

 Canon and the little White River are referred to it. 



The comparatively long-crowned teeth and nearly straight upper molars with their less rounded fossettes resemble 

 rather the teeth of Hipparion gratum (Leidy) than they do those of Protohippus perditus. The hitherto unknown foot 

 characters (Fig. 116a) are revealed in a referred specimen recently found by Troxell near Mission, South Dakota. The 

 manus is anisotridactyl, with a slender, laterally compressed median metatarsal. It resembles Mcrychippus rather than 

 Pliohippus. 



Protohippus niobrarensis Gidley 1906. 

 Plate 21.2. Text Fig. 108. 



Neohipparion niobrarense Gidley, Matthew, W. D., and Gidley, J. W. "New or Little Known Mammals from the Miocene of 

 South Dakota," Bull. Amer. Mus. Nat. Hist. Vol. XXII, Art. VIII, May 26, 1906, p. 151, figs. 18, 19, 20. 



Horizon and locality. — Near Ft. Niobrara, Niobrara River, Nebraska, Procamelus-Hipparion zone, Nebraska, Lower 

 Pliocene. Type collected by J. W. Gidley, American Museum Expedition of 1903. 



Type. — Amer. Mus. 10828, anterior portion of skull with lower jaws (Gidley, 1907) "The type specimen is of a 

 very old individual with the tooth characters practically obliterated by wear. The skull and jaw characters, therefore, 

 form the basis of its generic reference." Measurements: molar-premolar series, p'-m 3 , .117; p 4 a.p. .017, tr. .0215; m 1 

 a.p. .0165, tr. .0205. 



Type figure. — Text Fig. 108 of this Memoir. 



Characters. — (Gidley, 1906, p. 152, 1907, p. 927) Gidley separates this species from Hipparion (Neohipparion) 

 gratum as follows: (1) The skull, especially in the nasal and premaxillary region, is longer and narrower; (2) the palate is 

 narrow and more arched; (3) the anterior palatine foramina are more open but do not extend as far backward as in H. 

 gratum; the posterior palatal notch apparently extends as far forward as the middle of m 1 ; (4) the malar fossa is wanting; 

 (5) the lachrymal fossa is larger, extends farther backward, and has a rather sharply defined posterior border; (6) the 

 symphysis of the lower jaw is longer and narrower than in H. gratum but is not so extreme in proportions as in H. dolichops; 

 and (7) the ramus is bowed as in 77. gratum but has a less vertical depth. 



Protohippus simus Gidley 1906. 



Plates 21.1, 22.2. Text Fig. 109. 



Protohippus simus sp. nov., Matthew, W. D., and Gidley, J. W. "New or Little Known Mammals from the Miocene of South 

 Dakota," Bull. Amer. Mus. Nat. Hist. Vol. XXII, Art. VIII, May 26, 1906, p. 139, no figure. 



Horizon and locality. — - Little White River, near the Rosebud Indian Agency, South Dakota, Lower Pliocene. Pro- 

 camelus-Hipparion zone. Type collected by American Museum Expedition of 1902. 



Type. — Amer. Mus. 9820, anterior portion of adult skull with nearly complete dentition. Measurements: molar- 



