170 



osborn: OLIGOCENE, MIOCENE, pliocene equid.e. 



Type. — Univ. Texas Coll. (cast Am. Mus. 14392). (Cope) An inferior molar, ?m 2 , of the right side. Measure- 

 ments: (Cope) a.p. .0165, tr. .008, length of crown .037. 

 Type figure. — Text Fig. 136 of this Memoir. 



Characters. — (Cope) (1) Inferior molar narrowly hypsodont; (2) investment of cement thin; (3) metaconid and 

 metastylid appressed to hypoconid and protoconid, and spreading widely apart so as to be connected by an anteroposterior 

 isthmus; (4) hypostylid also appressed to the hypoconid. 



Gidley (1907, p. 921) regards this type as of uncertain reference with characters rather those of Protohippus and 

 Hipparion than of Equus. A series of lower molars (Amer. Mus. 10626) referred by Gidley may relate this species to 

 Protohippus. - 



Note. The type may belong to an Hipparion in view of the wide column and shallow gutter of inner wall. 



PLIAUCHENIA PLIOHIPPUS PROVERSUS ZONE. 18. UPPER PLIOCENE. 



This zone is typified in the Upper Etchegoin of the Coalinga region of California. 



Pliohippus proversus Merriam 1916. 

 Text Figs. 137, 138. 



Pliohippus proversus, n. sp., Merriam, John C. "Relationship of Equus to Pliohippus Suggested by Characters of a New Species 

 from the Pliocene of California," Univ. Cal. Publ. Bull. Dept. Geology, Vol. 9, No. IS, March 20, 1916, pp. 525-534, figs. 2, 13. 



Horizon and locality. — Upper Etchegoin, Pliocene, of the North Coalinga region, western border of the San Joaquin 

 Valley, California, locality 2079. (Merriam, p. 532) "In this connection it should be noted that the upper Etchegoin 

 stage, in which P. proversus occurs, is separated from the Pleistocene horizon containing E. occidentalis by the Tulare for- 

 mation, representing a thickness estimated by Arnold and Anderson 1 to include at least three thousand feet of strata." 



Type. — Univ. Cal. Pal. Coll. 21330. A superior molar, m 1 or m 2 , of the left side. Measurements: (Merriam) m 1 

 a.p. .030, tr. a.027, a.p. diameter of protocone .0138. Paratype and referred specimens figured but not precisely desig- 

 nated, from the same locality, a third superior premolar, p 3 , Univ. Cal. Pal. Coll. 22328, a fourth superior premolar, p 4 , 

 21331, an upper molar, m , 22329 (these inferior cheek teeth include premolars and molars); the astragalus and a portion 

 of the left metacarpal. 



Type figure. — Text Fig. 137 of this Memoir. 



Characters. — (Merriam, 1916, p. 526) "Cheek-teeth large, long-crowned, heavily cemented. Upper cheek-teeth 

 slightly curved; mesostyle heavy; fossettes wide to narrow, with moderately crinkled enamel borders; protocone large, 

 strongly compressed laterally in the molars, inner border convex or nearly flat. Lower cheek-teeth with short or long 

 parastylid; metaconid-metastylid column commonly long anteroposteriorly and narrow transversely, inner groove wide, 

 flat as in Equus, or somewhat narrowed tending toward the angular form seen in Pliohippus; outer faces of protoconid and 

 hypoconid either convex or somewhat flattened. Limb elements, so far as known, much like those of Equus. Unciform 

 facet of metacarpal III sloping away from the plane of the magnum at approximately the angle shown in Equus. Lateral 

 digits apparently much reduced distalfy and feet presumably monodactyle." 



Merriam observes (1916) that Pliohippus proversus is most nearly related to P. siiuplicidcns Cope and P. cumminsii 

 Cope of the Blanco, Middle Pliocene, of Texas. The molars of these three species are distinguished from those of the 

 genotype P. pernix Marsh by (1) straighter crowns; (2) heavier mesostyle; (3) narrowing of the fossettes; (4) protocone 

 relatively wide transversely in the premolar region but flatter or narrower transversely in all the cheek teeth than in P. 

 pernix; (5) especial resemblances of the referred lower teeth to those referred by Cope to P. simplicidens; (6) the crowns 

 large, long, and heavily cemented; (7) metaconid-metastylid column wide anteroposteriorly and the internal groove 

 broad and flat. 



1 Arnold, Ralph, and Anderson, Robert, U. S. Geol. Surv. Bull. 398, 1910, p. 147. 



