PERISSODACTYLA 179 



PERISfSODACTYLA 

 BY J. W. GIDLEY 



Due to modern expert methods of collecting, the past few years have 

 added very greatly to the collections in our various museums, both in 

 number and quality of specimens representing the Perissodactyla. The 

 various field parties sent out by the American Museum of Natural His- 

 tory have been very fortunate in bringing together many skeletons and 

 good skull material, especially of the horses, rhinoceroses, and Titano- 

 theres. Other expeditions, especially those of the Carnegie Museum, the 

 University of Nebraska, and Amherst University, have also added many 

 new and valuable specimens to the general store. The study of this better 

 material, together with the revision of the old, by Osborn, Matthew, Gid- 

 ley. Granger, and others, has marked a very considerable advance in our 

 knowledge of the Perissodactyla. 



Notwithstanding the good results reached through recent extensive 

 studies, aided by this new and more complete material collected in recent 

 years, the classification of the order is still in an unsettled state and many 

 other problems await the discovery of more and better material to work 

 out their solution. Thus it is not possible as yet to give an altogether 

 satisfactory classification, or even to define the families and subfamilies 

 in such a way that some objection would not be made by one authority or 

 another. Osborn in his latest classification^ divides the order into four 

 superf amilies and eight families, as f ollow^s : 



I. TiTANOTHERioiDEA, including two families, the Pala'osyopidce 

 and Tiianotlicriidce (all extinct). 

 11. HiPPOTDEA, including the two families Equidce and Paleotheriidce. 



III. Tapiroidea, including the two families Tapir idw and Lophio- 



dontidce. 



IV. Rhinoceeotojdea, including the two families Hyracodontidfu 



and Rhino cerotidce. 



Under these 8 ■ families, 74 genera are included. Up to the present 

 time about 500 species of Perissodactyls, living and extinct, have been 

 recognized. Of these about 475 are extinct, and future explorations in 

 the fossil fields will doubtless add many more to this number. Thus it 

 seems evident that the order has long since reached its zenith of develop- 

 ment and is now on the rapid decline. The extinction of the Titanotheres 

 seems to have been accomplished before the close of the Oligocene, but 



1 The Age of Mammals, 1910, p. 555. 



