CONTRIBUTIONS TO THE KNOWLEDGE OF THE MAM- 

 MALS OF THE PLEISTOCENE OF NORTH AMERICA. 



By Oliver P. Hay, 



Research Associate of the Carnegie Institution of Washington. 



INTRODUCTION. 



The results detailed on the following pages have been obtained in 

 the course of the writer's investigations on the Vertebrata of the 

 Pleistocene epoch in North America. These results include descrip- 

 tions of two extinct horses, one new extinct bison, one new and one 

 already- described musk ox, measurements of certain limb bones of 

 fossil horses, and discussion of the meaning of the variations observed. 

 Also there have been secured measurements from many skulls ^of 

 various equids, as of Przevalsky's horse, of a number of fossil horses, 

 of many domestic horses, of three species of zebras, of the domestic 

 ass, of the chigetai (Equus hemionus)^ and of the kiang (E. kiang). 

 A considerable number of these measurements have been quoted from 

 other authors, but many have been made by the writer on skulls 

 which belong to the United States National Museum and to the 

 American Museum of Natural History, and on a fossil skull in the 

 University of Kansas. From these measurements certain indices in 

 use in equine craniometry have been computed. From these meas- 

 urements and indices an attempt has been made to determine to what 

 extent the various unmixed and wild species which are considered 

 deviate from an average condition; likewise, to ascertain the value 

 of some of the measurements and indices which have been employed 

 in the study of domestic horses; and, finally, an endeavor has been 

 made to throw some light on the elements which have contributed 

 to the formation of that assemblage of horses which bears the name 

 Equus caballus. 



The writer acknowledges his obligations to the institutions which 

 have kindly and freely granted him access to their materials. 



1. DESCRIPTIONS OF A BISON AND TWO MUSK OXEN. 

 BISON SYLVESTRIS, new species. 



Diagnoses, — A bison of the late Pleistocene, having the horn cores 

 feeble, even in case the type was a female, and apparently directed 

 outward in a plane at right angles with the axis of the skull. Ptery- 



Proceedinqs U. S. National Museum, Vol. 48-No. 2086. 



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