48 



osborn: OLIGOCENE, MIOCENE, pliocene equid^e. 



Type. — Amer. Mus. 674, a complete superior grinding series, p'-m 3 of the right side, adult. Paratype, Am. Mus. G73, 

 an immature maxilla containing deciduous premolars and first molar, dp 1 " 1 , m 1 . Measurements may be taken from 

 type figure. 



Type figure. — Plate 2.6, text Fig. 27 of this Memoir. 



Characters. — The specific name M. trigonostylus refers to the progressive triangular form of the plications of the hypo- 

 style, which is highly distinctive. From the rudiment of the hypostyle on the post-eingulum, as seen in the M. bairdii 

 stage, a cruciform plication extends anteriorly toward the metaloph, branching into an external pli hypostyle and an 

 internal spur (see paratype, dp 4 , Plate 2.5) which connects with the metaloph. This condition of the hypostyle is very 

 different from that in any other Middle Oligocene species (compare Plate 2.1-8). A somewhat similar condition of the 

 hypostyle is seen in the type of Miohippus validus from the Protoceras zone. 



Fig. 27. (6) Type of Mesohippus trigonostylus Osborn, Amer. Mus. 674, adult dentition. (5) Amer. Mus. 673, 

 paratype, milk dentition. Both figures natural size. 



Mesohippus obliquidens Osborn, fQ04. 

 Plates 1.5, 2.7. Text Fig. 28. 



Mesohippus obliquidens, sp. nov., Osborn, Henry F. "New Oligocene Horses," Bull. Amer. Mus. Nat. Hist., Vol. XX, Art. XIII, 

 May 28, 1904, p. 173, fig. 4, PI. v, C. 



Horizon and locality. — Cheyenne River, Big Badlands of South Dakota, "from the nodular layer in lower middle 

 layer of the Oreodon Beds; specimen covered with ferruginous oxide." Type collected by Amer. Mus. Expedition of 

 1892. Lower Oligocene, Middle Oreodon zone. 



Type. — Amer. Mus. 668, skull and jaws. A young individual with three milk teeth in situ. Measurements: lower 

 jaw from condyle to symphysis .149; m 1 a.p. .013, tr. .015. 



Type figure. — Text Fig. 28 of this Memoir. 



Characters. — (Osborn, 1904) "Distinguished from the other species of the Oreodon Beds by (1) large size of skull 

 and teeth as compared with M. bairdii, (2) preorbital [lachrymal] fossa shallow or wanting, (3) but especially by the high 

 crowns of upper and lower molars: ectoloph of m 1 measures .0105 vertical as compared with .008 in an unworn m 1 crown of 

 M. bairdi[i], (4) metaloph elevated, metaconule not being clearly defined, (5) proto- and metalophs directed obliquely back- 



