Soutb Dakota School of Mines 89 



All rhinoceroses, living and extinct, are divided by Osborn 

 into three subdivisions, as follows :* The Hyracodontidae or 

 cursorial (upland) rhinoceroses; the Amynodontidae or aquatic 

 rhinoceroses, and the" Rhinocerotidae or true (lowland) rhinoc- 

 eroses. Of these the first two are found only in the fossil state, 

 the third is found both fossil and living. In America, the cur- 

 sorial rhinoceroses are found first in the Middle Eocene, the 

 c quatic rhinoceroses in the Upper Eocene, and the true rhinocer- 

 oses in the Lower Oligocene. The first two became extinct here in 

 the Oligocene, but the true rhinoceroses endured until after the 

 close of the Miocene. All three occur in fossil form within the 

 area described in this paper, the cursorial and aquatic species 

 in the Oligocene, chiefly in the Middle Oligocene, the true rhi- 

 noceroses 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 Hy- 

 racodonts were small, light chested, swift footed, hoofed, horn- 

 less creatures, much resembling the Miocene horses and evi- 

 clenty well-fitted for living on the grass-covered higher lands. 

 The Amynodonts were heavily built, short-bodied, hornless ani- 

 mals, 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 pre- 

 hensile upper lip, apparently tending toward proboscoid develop- 

 ment. The animal evidently much resembled the present day 

 hippopotamus, both in build and in habit (see Plate 29). One 

 adult skeleton, that of Metamyno don planifrons in the Ameri- 

 can Museum of Natural History, measures nine and one-half 

 feet long and four and one-half feet high at the shoulders. The 

 true rhinoceroses began as light limbed, hornless animals, in- 

 termediate in proportion between the two just mentioned, and 

 in size and structure not greatly unlike the modern tapirs. Dur- 

 ing much of their early life history they, like the more primitive 

 Hyracodonts and Amynodonts, were entirely without horns. 



The true rhinoceroses constitute in many respects the most 

 important of the three subdivisions and to the paleontologist 

 are of profound interest. They lived in great abundance in the 

 region of the Black Hills during Oligocene and Miocene time, 

 and their skeletons in certain favored localities, particularly in 

 the Big Badlands and in Sioux County, northwestern Nebraska, 

 have been collected in large numbers. The Oligocene forms 



*Oisiborn, H. P. The Extinct Rhinoceroses. Mem. Am. iMus. Nat. 

 Hist., Vol. 1, 1898, ip.p. 75-164, pis. Xlla-XX. 



