PLATE 6 



PRE0RB1TAL FOSSAE IN OLIGOCENE AND MIOCENE EQUID.E FROM OREGON, COLORADO, SOUTH DAKOTA 



AND NEBRASKA. 



(Left column) Comparison of front of skull in Miohippus, Archaeohippus, Kalobatippus, and Hypohippus. 

 (Right column) Comparison of front of skull in species of Parahippus and Merychippus. The lachrymal fossa is 

 broad and comparatively shallow in the later species. The malar fossa is not differentiated. 



All figures one-half natural size. 



Left column. 



Middle John Day, Ore. Upper Oligocene 1. Miohippus equiceps, type, Am. Mus. 7261. Lachrymal 



fossa broad, shallow; malar fossa not differentiated. 

 Mascall, Ore. Middle Miocene 2. Archaeohippus ultimas Cope, type, Am. Mus. 8174. Lachry- 



mal and malar fossae deep, well differentiated. 

 Upper John Day, Ore. Uppermost Oligocene, 3. Kalobatippus prwstans Cope, type, Am. Mus. 7269. Lachry- 



transitional to mal fossa broad, rather shallow; malar fossa not differen- 



Lower Miocene tiated. 

 Pawnee Creek, Col. Middle Miocene 4. Hypohippus osborni Gidley, type, Am. Mus. 9407. Lachry- 



mal fossa deep, contracted; no malar fossa. 



Lower Rosebud, Pine Ridge, Lower Miocene 



So. Dak. 

 Upper Harrison, Sioux Co., 



Nebr. 



Sheep Creek, Sioux Co., Nebr. Middle 

 Mascall, Ore. 



Right column. 



5. Parahippus pristinus Osborn, type, Am. Mus. 12918. 



7. P. alius Marsh, referred specimen, Am. Mus. 14182 



(reversed in drawing). 



8. Merychippus isonesus Cope, type, Am. Mus. 8175. 



6. P. ncbrascensis primus Osborn, type, Am. Mus. 13770. 



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