O. C. Marsh — Eastern Miohippus Beds. 91 



Art. XII. — Eastern Division of the Miohippus Beds, 

 with Notes on some of the Characteristic Eossils / by 

 O. C. Marsh. 



Lsr 1871, the writer .explored the Miocene deposits of 

 Oregon, especially along the valley of the John Day river, 

 and these were again examined with more care in 1873. The 

 strata were found to be nearly a mile in thickness, and 

 deposited in a single lake-basin, which was subsequently named 

 the John Day basin, from the river that now drains it.* 

 The upper portion of these Miocene deposits represented 

 a distinct horizon, and was named by the writer the Miohippus 

 beds, from one of the most characteristic genera discovered 

 in it.f Among other ungulate mammals likewise obtained 

 from these strata were Dicer atheriam, a new genus of the 

 Rhinoceros family, and Thinohyus, a suilline form allied to 

 the existing peccaries. Subsequent researches brought to light 

 other interesting fossils in this horizon, which has since been 

 supposed to be represented only on the Pacific coast. 



The Miocene strata on the eastern slope of the Rocky moun- 

 tains have long been recognized in two distinct horizons, the 

 lower known as the Titanotherium, or Brontotherium, beds, 

 from the huge mammals which they contain, and above these, 

 the Oreodon beds, of which that genus is characteristic. 



Various vertebrate fossils have been obtained from time to 

 time in the eastern Miocene deposits, which were not known 

 to occur in either of the two horizons, but only of late have the 

 uppermost strata been recognized as distinct from the Oreodon 

 beds on which they rest. The horizon thus indicated has 

 been named by Wortman the Protoceras beds, from a most 

 remarkable genus, Protoceras, recently found in them, and 

 described by the writer.:]; 



An examination of material from this horizon, recently 

 made by the writer, brought out the interesting fact, that 

 the genus Miohippus is one of its characteristic fossils, and 

 that the type species, M. annectens, Marsh, described in 

 1871 from the Oregon beds, is present. Diceratherium, 

 Thinohyus, and other genera typical of the western strata, are 

 also found in the eastern, so that it is now demonstrated 

 that the Miohippus horizon has an eastern as well as a 

 western division, a fact of much scientific interest. Doubtless 

 each division will be found to contain certain forms peculiar 

 to itself, even if all are contemporaneous, a question which 

 future discoveries must decide. 



* This Journal, vol. ix, p. 52, January, 1875. 

 f Ibid., vol. xiv, p. 355, November, 1877. 

 \ Ibid., vol. xli, p. 81, January, 1891. 



