102 



OSBORN: OLIGOCENE, MIOCENE, PLIOCENE EQUIDjE. 



Type. — Amer. Mus. Cope Coll. 8175. A nearly complete skull, cervical vertebrae, pelvis and hind limbs. Measure- 

 ments: (Cope, 1S89, p. 453) incisors to occipital condyles .328; m 1 a. p. .019, tr. .019; length of Mts. Ill .175. 

 Type figure. — Text Fig. 75 of this Memoir. 



Characters. — (Gidley, 1907, p. 909, Osborn 1918) (1) Molars comparatively short-crowned, of true Merychippus 

 type; (2) protocone and hypocone subequal in size, elongate-oval, with internal spurs approaching protoconule and 



Fig. 75. Original figure of the teeth of the type of Merychippus isonesus Cope, Am. Mus. Cope Coll. 8175. Natural 

 size. After Cope, 1889, fig. 23. 



metaconule respectively; (3) hypocone early joining metaconule; (4) protocone separated half way down the crown; 

 (5) median crest of metaconule and crochet region ptychoid; (6) a pli caballin. (7) Facial region relatively short; (8) a 

 single lachrymo-malar fossa, not well separated. 



Matthew (1913) regards this type as rather close to that of M. paniensis with some valid specific differences. The 

 median metapodials (Mts. Ill) and the laterals (Mts. II, IV) are less produced than in M. paniensis. 



Merychippus isonesus primus mut. nov. 

 Plates 13.3, 14.2,4, 25.4, 39.2. Text Fig. 78. 



Fig. 7(3. Geologic distribution in ascending order of mutations of Merychippus isonesus 



from Sheep Creek. 



Amer. Mus. 14184, type of M. isonesus quartus Osborn; Amer. Mus. 14185, type of M. isonesus quintus Osborn; 

 Am. Mus. 14181, specimen referred to M . isonesus quartus Osborn; Am. Mus. 141S0, type of M. isonesus tcrtius Osborn; 

 Am. Mus. 14179, type of M . isonesus sccundus Osborn; Am. Mus. 14187, type and topotype of M. isonesus primus Osborn, 

 the most primitive form. The section represents one of the gullies in the Sheep Creek beds, at the type locality. Near 

 the.top of the Sheep Creek is a three-foot bed of grey volcanic ash. A little above are the unconsolidated sands of the 

 Snake Creek horizon. 



Horizon and locality. — Sheep Creek phase of Arikaree formation, western Nebraska, lower Middle Miocene. Type 

 collected by American Museum Expedition of 1908. 



Type. — Amer. Mus. 14187. Upper grinding teeth, p 2 -m 3 . 



