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the fossil state, the third is found both fossil and living. In 

 America, the cursorial rhinoceroses are found first in the 

 Middle Eocene, the aquatic rhinoceroses in the Upper 

 Eocene, and the true rhinoceroses in the Lower Oligocene. 

 The first two became extinct here in the Oligocene, but the 

 true rhinoceroses endured until after the close of the Mio- 

 cene. All three occur in fossil form within the area described 

 in this paper, the cursorial and aquatic species in the Oligo- 

 cene, chiefly in the Middle Oligocene, the true rhinoceroses 

 throughout both the Oligocene and the Miocene. 



The three families differed greatly from one another, 

 both in exterior form and in dental and skeletal structure. 

 The Hyracodonts were small, light chested, swift footed, 



Figure 36 — Skeleton of the small, swiftfooted Oligocene rhinoceros, 

 Hyracodon nebrascensis. Osborn, 1898. 



hoofed, hornless creatures, much resembling the Miocene 

 horses and evidently well-fitted for living on the grass- 

 covered higher lands. (Plates 30 and 38). The Amyno- 

 donts were heavily built, short-bodied, hornless animals, 

 with spreading padded feet, four functional toes in front, 

 eyes and nostrils much elevated supposedly for convenience 

 in swimming, canine teeth enlarged into recurved tusks, 

 and a prehensile upper lip, apparently tending toward 

 proboscoid development. (Plates 29 and 30). The ani- 

 mal evidently much resembled the present day hippo- 

 potamus, both in build and in habit. One adult skeleton, 



