SOUTH DAKOTA SCHOOL OF MINES 91 



tional one. The tapir, an anatomically unprogressive crea- 

 ture, is a partial exception in that it has four toes on the 

 front foot and three toes on the hind foot. Similar excep- 

 tions or seeming exceptions occasionally existed in the evolu- 

 tionary development of other perissodactyls, nevertheless the 

 bisection of the third toe by the median plane of the foot 

 early asserted itself and has continued with firm persistence. 



Existing perissodactyls include animals of greatly dif- 

 fering appearance and habits but their skeletal characters 

 indicate with certaintly their relationship and skeletal 

 characters indicate also the wide gap between them and 

 other hoof-bearing creatures. 



The perissodactyls constitute a restricted group and 

 although many prehistoric forms are known — in all about 

 five hundred species — living species are confined to the three 

 well known families, rhinoceroses, tapirs, and horses. Of 

 fossil forms the following families are represented in the 

 White river badlands : Titanotheridae, Equidae, Tapiridae, 

 Lophiodontidae, Hyracodontidae, Amynodontidae, and Khin- 

 ocerotidae. 



The living forms so far as concerns their present 

 natural habitat, with the exception of the American tapirs, 

 are all confined to the Old World. Gidley calls attention to 

 the fact that this is the more interesting since North America 

 seems to have been the birth place or at least the stage for 

 the development, not only of the early representatives of all 

 the living Perissodactyls, but of most of the extinct groups 

 of the order as well and that half the total number of 

 perissodactyl species described have been founded on speci- 

 mens from the Tertiary and Quaternary formations of this 

 country. 



RHINOCEKOTOIDEA 



The finding of fossil bones of true rhinoceroses in the 

 Big Badlands by Alexander Culbertson in 1850, and their 

 prompt and accurate identification by Leidy, constitute one 

 of the most interesting, unexpected, and instructive paleon- 

 tological discoveries of America. 



Existing rhinoceroses are confined to Africa, the Indian 

 Archipelago and the southern parts of Asia. These form 

 but a small representation of the numerous ancestry that 

 abounded in North America from Middle Eocene to late 



