MIOHIPPUS. 



5] 



GENUS MIOHIPPUS Marsh, 1874. 

 Amer. Jour. Sri. and Art*, (3), Vol. VII, Mar. 1874, pp. 249-250. 



Genotype. — Miohippus annectens Marsh, type probably from Upper John Day, Oregon. 



The generic stage Miohippus and the related Kdlobatippus embrace twenty species described between 1871 and 1918, 

 including six species transferred from Mesohippus, six species transferred from Anchitherium, as shown in the table below. 



As will be observed in a comparison of Plates 1-3, the line between such species as Miohippus (Mesohippus) 

 meteulophus and M. brachystylus of the Leptauchenia zone and M. (Mesohippus) intermedins of the Protoceras zone is 

 purely arbitrary. It is obvious that members of more than one phylum are passing from one genus into the next, and 

 Mesohippus meteulophus and M. brachystylus may with equal consistency be referred to Miohippus. 



The geologic range of Miohippus is through the Protoceras zone of the Big Badlands of South Dakota, probably also 

 certain levels of the Leptauchenia zone in the same region, into the Diceratherium zone or Middle John Day of Oregon 

 and up into the Promerycoehoerus zone of the Upper John Day of Oregon also into the Lower Rosebud formation 

 (Miohippus equinanus, M. gemmarosee) of Pine Ridge, South Dakota. The various species have not as yet been compared 

 in sufficient detail to separate the phyla; in fact, the species themselves require much condensation. 



The chief stages of character genesis and evolution and of proportioned evolution observed in the fifteen species at present 

 referred to Miohippus are as follows: 



1. Premolars ll| typically exceed in length molars pf. 



2. P 4 typically exceeds m 1 in width. 



3. Cranial exceeds facial region in length- (M. intermedins, M. validus, tM. gidleyi); facial exceeds cranial region in 

 length in M. meteulophus, M. brachystylus, also in all more progressive stages of the Equidse. 



4. Grinding teeth with internal cingulum absent or reduced (M. gidleyi). 



5. Lophs of the bunolophoid or lophoid types. 



6. Hypostyle large, typically angular, more or less separate from post-cingulum. 



7. A rudimentary crochet in M. gidleyi and M. equinanus. 



8. A relatively deep lachrymal fossa or a relatively broad, shallow lachrymo-malar fossa. 



9. Pes more progressive, a Mts. Ill-cuboid facet. 



LEPTAUCHENIA-PROTOCERAS-MIOHIPPUS ZONE. 5. UPPER OLIGOCENE. 



as described in the geologic section, this important and distinct zone is distributed in south dakota, 

 North Dakota, and Montana. It is distinguished by the presence of a variety of species, all of which 

 are referred to the mlohippus stage. 



Miohippus meteulophus Osborn, 1904. 



Plates 1.7, 3.1. Text Fig. 30. 



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

 May 28, 1904, pp. 167-179, figs. 1-S, PI. v. pp. 174-5, PL v, D. 



Horizon and locality. — Big Badlands of South Dakota, Leptauchenia zone, Upper Oligocene. Type collected by 

 Am. Mus. Expedition of 1894. 



Type. — Amer. Mus. 1210, adult skull and jaws. Measurements: basilar length, occipital condyles to premaxillaries 

 .215; p'-m 3 .078; m 1 " 3 .038; m 1 a.p. .013, tr. .017. 



Type figure. — Text Fig. 30 of this Memoir. 



Characters. — (Osborn, 1904, p. 174) "This species is a successor of M. culophus, from which it is distinguished (1) 

 by its decidedly more elongate or hypsodont molar crowns, the crests being .002 higher; (2) by its larger size; (3) unlike 



