148 



osborn: oligocene, Miocene, pliocene equid.e. 



Matthew observes that certain species of Pliohippus approach the South American Pampean genera Hippidium and 

 Onohippidium. Osborn considers that they are excluded from direct ancestry by certain characters, such as the deep 

 malar fossse. The known species of North American Pliohippus fall into the following partly progressive and partly 

 phyletic stages and groups: 



Group I, Upper Miocene. 



Species with short-crowned teeth, approximating Mcry- 

 chippus, 



PI iohippus mirabilis, 



Group II, Upper Miocene and Lower Pliocene. 



Long-crowned molars, heavy cement on both deciduous and 

 permanent grinders. 



Pliohippus pernix 



" suprcmus 

 " robustus 



Group III, Power and Middle Pliocene. 



Larger, long-crowned teeth, with broad fossettes, simple 

 borders. 



Pliohippus spectans 

 " inter polalus 

 " nobilis 



Group IV, Middle Pliocene. 



Crowns approaching Equus pattern. 

 PI ioh ip pus simpliciden s 

 " cumminsii 

 " proper sus 



Fig. 117. Original figures of the type of Pliohippus mirabilis Leidy, U. S. Nat. Mus. 569. After Leidy 1869, pi. 

 VII. (10) External view of fragment of type maxilla showing dp 3 , 4 ; (11) crown view of dp 3 , 4 ; (12) crown view of m 1 

 removed from the matrix; (13) side view of m 1 . Natural size. 



PROCAMELUS HIPPARION PLIOHIPPUS SUPREMUS ZONE. 13. LOWER PLIOCENE G. 32. 



The typical exposures of this zone are on the Niobrara River, but it is also widely distributed in other 

 parts of the western united states. 



Pliohippus mirabilis Leidy 1858. 



Plates 25.10, 20.3. Text Fig. 117. 



Merychippus mirabilis, Leidy, Joseph. "Notice of Remains of Extinct Vertebrata, from the Valley of the Niobrara River, 

 collected during the Exploring Expedition of 1857, in Nebraska, under the command of Lieut. G. K. Warren, U. S. Top. Eng., by Dr. 



