170 



OSBORN: OLIGOCENE, MIOCENE, PLIOCENE EQTTIDjE. 



Type. — Univ. Oregon Coll. 672. Two lower grinding teeth, p 4 , mi. Measurements: p 4 a. p. .021, tr. .0111, meta- 

 conid-metastylid column .0126, height of crown, .031; mi a.p. .0176, tr. .0103, metaconid-metastylid column a. p. .0112. 

 Type figure. — Text Fig. 139 of this Memoir. 



Characters. — (Merriam) (1) Length of crowns similar to advanced stages of Mcrychippus; (2) heavily cemented; 

 (3) metaconid-metastylid column more elongate anteroposteriorly than in any Merychippus form known; (4) wide and 

 flat internal groove between metaconid and metastylid, typical of Hipparion; (5) parastylid extends inward as far as meta- 

 conid; (6) a sharp anteroexternal ridge developed on protoconid. 



Merriam observes that the open portions of the internal valleys correspond approximately with those seen in Hip- 

 parion, and that the progressive stage seems rather that of Hipparion than of Mcrychippus. 



A referred specimen is Univ. Oregon Coll. 668, the extremity of a lower jaw containing the lower incisors. 



PROCAMELUS HIPPARION ZONE. 13. LOWER PLIOCENE. 



This zone is typified in the Fort Niobrara, Nebraska, and the Little White River of Nebraska and Dakota. 



Hipparion occidentale Leidy 1S56. 



Plates 25.9, 34.5. Text Fig. 140. 



Hipparion occidentale, Leidy. "Notices of some remains of extinct Mammalia, recently discovered by Dr. F. V. Hayden, in the 

 bad lands of Nebraska," Proc. Acad. Nat. Sci. Phila. Vol. VIII, No. 1, 1856, p. 59, no figure. "The Extinct Mammalian Fauna of 

 Dakota and Nebraska," Jour. Acad. Nat. Sci. Phila. (2) Vol. VII, 1869, pp. 281-282, PI. xviii, figs. 1-5. 



Horizon and locality — (Leidy, 18.56.) "... .the White River of Nebraska." (1869, p. 281) "... .the Mauvaises 

 Terres of White River, Dakota. . . .According to Dr. Hayden, they belong to the superficial portion of the tertiary deposits, 

 or bed F of the section, as indicated on p. 16, and which is more fully developed on the Niobrara River and at Bijou Hill." 

 Type collected by F. V. Hayden. (Gidley, 1907) Little White River, South Dakota, "Nebraska formation," Lower 

 Pliocene. 



Type, — Acad. Nat. Sci. Phila. Coll. (Leidy, 1869, p. 281) "The teeth consist of four upper molars of the right, and one 

 of the left side, and are represented in figures 1-5, plate XVIII. They were accompanied by a fragment of a last upper 

 molar and a lower molar, apparently of another equine animal. The specimens are between a third and a half worn away, 

 and exhibit on their triturating surface the characters usually ascribed to the genus." (Gidley, 1907, p. 877) Four upper 

 teeth apparently of a single individual (right) p 2 , p 3 , and m 2 , and (left) p 3 , No. 3, Acad. Nat. Sci. Phila. Coll., cast Amer. 

 Mus. 10794. Measurements, (Gidley, 1907) p 2 a.p. .0315, tr. .0235; p 3 a.p. .0205, tr. .025; m 2 .023, tr. .022; a.p. 

 diameters of protocones, p 2 .008, p 3 .010, m 2 .008. 



Type, figure. — Text Fig. 140 of this Memoir. 



Characters. — (Leidy, 1856, 1869, Gidley, 1907) (1) Protocone isolated, elliptical, anteroposterior diameter twice as 

 great as transverse; (2) protoconule, metaconule, and fossette borders of upper grinders comparatively complicated but 

 less so than in European types; (3) hypocone small, about one-half the size of the protocone; (4) upper premolars relatively 

 elongate anteroposteriorly; (5) upper molars compared with premolars comparatively small; (6) tooth crowns long and 

 slightly curved; (7) protocone of uniform width throughout its length; (8) a much larger animal than Hipparion vennstum 

 of South Carolina, approaching the H. gracile of Europe, and resembling in size, proportions, and curvature the teeth 

 of Equus asinus. 



A referred specimen is Amer. Mus. 10579. 



