PLATE 1. 



SKULLS OF MESOHIPPUS AND MIOHIPPUS FROM THE LOWER, MIDDLE, AND UPPER OLIGOCENE. 



All figures natural size. 



The Mesohippus crania are arranged in ascending systematic and geologic order and exhibit progressive increase in 

 size from the small skulls of the Titanotherium zone to the larger skulls, such as the juvenile M. obliquidens, of the Middle 

 Oreodon zone. Indications of the preorbital fossse are observed in the smallest and most ancient of these horses. 



Left column 



Cheyenne River, So. Dak. Lower Oligocene 1. Mesohippus proteulophus Osborn, referred specimen, Am. Mus. 1208 



(reversed in drawing). 



" " " " 2. M. hypostylus Osborn, type, Am. Mus. 1180 (partly reversed in 



drawing) . 



? ? ? ? ? ? 3. M. bairdii Leidy, type. Mus. Cat. No. not known. Type lost. 



" " " " " " 4. M. bairdii Leidy, neotype, Am. Mus. 1477 (partly reversed in 



drawing). 



" " " " Middle " 5. M. obliquidens Osborn, type, Am. Mus. 688 (reconstructed in 



drawing). 



Cedar Creek, Col. 6. M. eulophus Osborn, type, Am. Mus. 8791 (partly reversed in 



drawing). 



Right column. 



7. Miohippus meteulophus Osborn, type, Am. Mus. 1210. 



8. M. brachystylus Osborn, type, Am. Mus. 11860. 



9. M. intermedins Osborn and Wortman, type, Am. Mus. 1196. 



10. M. gidleyi Osborn, type, Am. Mus. 1192. 



11. M. validus Osborn, type, Am. Mus. 1218. 



The smallest of the Miohippus crania exceeds in size the largest crania of Mesohippus from the Upper Oreodon zone. 

 The Miohippus crania exhibit a progressive increase in size. The three skulls from the river channel sandstones of the 

 Protoceras zone, namely, M. intermedins, M. gidleyi, M. validus, are believed to be of the same geologic age as the lower 

 levels of the Leptauchenia zone. 



Cheyenne River, So. Dak. Upper 



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