PLATE 2 



UPPER TEETH OF MESOHIPPUS FROM THE LOWER AND MIDDLE OLIGOCENE. 

 All figures natural size except 11 which is twice natural size, and 13 which is one-half natural size. 



Jefferson County, Mont. 

 Cheyenne River, So. Dak. 



Lower Oligocene 1. 



Middle 



2_ 

 3. 

 4. 

 5. 

 6. 

 7. 

 8. 



Left column. 



Mesohippus montanensis Osborn, type, Am. Mus. 9662 (reversed 

 in drawing). 



M. profeulophus Osborn, type, Am. Mus. 514a (reversed in drawing). 



M. hypostylus Osborn, type, Am. Mus. 1180. 



M. bairdii Leidy, neotype, Am. Mus. 1477. 



M. trigonostylus Osborn, paratype, Am. Mus. 673. 



M. trigonostylus Osborn, type, Am. Mus. 674. 



M. obliquidens Osborn, type, Am. Mus. 668. 



M. eulophus Osborn, type, Am. Mus. 8791. 



Western Nebraska? Lower 

 Cypress Hills, Assiniboia, Canr " 

 Cheyenne River, So. Dak. 



Right column. 

 9. M. celer Marsh, holotype, Yale Mus. 1 1302. 



10. M. westoni Cope, type, Ottawa Mus. 6289. 



11. M. ? var., referred specimen, Am. Mus. 9326, indet* 



12. M. ? sp. referred specimen, Mus. Comp. Zool. 6547. 



13. M. bairdii, specimen and record indet. (Composite drawing from 



different individuals). 



Dental characters of the Mesohippus stage are observed in the evolution from Mesohippus montanensis of the Lower 

 Titanotherium zone through seven specific stages of ascending mutations to Mesohippus eulophus of the L T pper Oreodon 

 zone. Note the gradual evolution of the hypostyle as a bud from the posterior cingulum, attaining a permanent 

 triangular form in M. trigonostylus of the Metamynodon sandstones, which are channel deposits contemporary with the 

 Lower Oreodon clays. Thus the M. bairdii stage and M. trigonostylus stage are believed to be nearly contemporaneous. 



* This progressive molar series is recorded from the Upper Oreodon zone of Pawnee Buttes; the correlation is somewhat 

 uncertain. 



226 



