100 



osboen: OLIGOCENE, MIOCENE, PLIOCENE EQUIMJ. 



All the evolutionary stages in the crown pattern of the grinding teeth of M cry chip pus may be followed in Plates 

 11 to 20, from the lower Middle Miocene of Sheep Creek (M. isonesus primus) through the Middle Miocene into the 

 Upper Miocene or Lower Pliocene (M. calamarius, M. republicanus). The key to this crown pattern is presented in 



TICHOLEPTUS-MERYCHIPPUS ZONE. 9. MIDDLE MIOCENE. 



This zone is typified in the Mascall of Oregon. It contains an abundant Merychippus fauna mingled 



WITH PROGRESSIVE SPECIES OF PaRAHIPPUS. 



Hippotherium relictum sp. nov., Cope, E. D. ' An Intermediate Pliocene Fauna," Airier. Naturalist, Vol. XXIII, No. 268, Apr. 

 18S9, pp. 253-254, no figure. "A Review of the North American Species of Hippotherium," Proc. Amcr. Philos. Soc. Vol. XXVI, 

 1889 (sig. dated June 5, 1889), p. 449, figs. 19, 19a, 20. 



Horizon and locality. — (Cope, Am. Nat. 1889, p. 253) " . . .a lake deposit in Oregon containing Canis, Elephas or 

 Mastodon, Holomeniseus or Auchenia, Aphelops, Equus." May be regarded as representing the Middle or Lower Plio- 



Fig. 73. Original figures of the type of Merychippus rclictus Cope, Am. Mus. Cope Coll. 8673, ?m 1 , 3 . Natural size. 

 After Cope, 1889, figs. 19, 19a, 20. 



cene. (Cope, 1S89) "From a Lower Pliocene bed (? Idaho terrane) of the eastern part of Oregon." (Gidley, 1907, p. 

 906) "Horizon not known, but probably Middle or Lower Miocene." Type collected by George C. Duncan. 

 (Matthew, 1915) Duncan's collection consisted in part of Pleistocene species, black, hard, resembling the Silver Lake 

 material in preservation. The remainder is softer, light buff colored, and consists wholly of Miocene species. Merriam 

 thinks the Miocene material may have come from out-lying beds of the Mascall. Possibly so, but my impression is that 

 the rhinoceroses and camels are later than Mascall. There may be three horizons mixed, Mascall, Rattlesnake, Silver 

 Lake." 



Type. — Amer. Mus. Cope Coll. 8673. (Cope, Am. Nat. 1889, p. 254) " Represented by two superior and three inferior 

 molar teeth." Measurements: (Gidley, 1907, p. 906) m 1 a. p. .016, tr. .018, height of crown .020 outside, .014 inside; 

 m 3 a. p. .0165, tr. .0165, height of crown .016 outside, .012 inside. 



Type figure. — Text Fig. 73 of this Memoir. 



Characters. — (Cope, Am. Nat. 1889, p. 254, Osborn, 1918) (1) Grinding surface nearly square; (2) the partly worn 

 crown relatively short, moderately curved ; (3) section of internal style (protocone) a wide oval, disconnected with hypo- 

 cone and hypostyle; (4) hypocone extending as far inward as protocone; (5) median branch of metaconule a posterior 

 spur of protoconule, ptychoid, a pli caballin, abundant cement. (6) About the size of Merychippus sevcrsus and resembling 

 it in many details. 



Figs. 2, 3. 



Merychippus relictus Cope 1889. 



Plate 13.9, 17.2. Text Fig. 73. 



