^ PROCEEDINGS OF THE UNITED STATES NATIONAL MUSEUM 



by the 



SMITHSONIAN INSTITUTION 



U. S. NATIONAL MUSEUM 

 Vol. 83 Washington : 1936 No. 2985 



A STUDY OF THE FOSSIL HORSE REMAINS FROM THE 

 UPPER PLIOCENE OF IDAHO 



By C. Lewis Gazin 



Assistant Curator, Division of Vertebrate Paleontology, United States National 



Museum 



INTRODUCTION 



The record of fossil horses in North America is one of the most 

 interesting and best known of the groups of mammals. The develop- 

 ment of the horse through Tertiary time appears to have been most 

 nearly continuous on this continent, stages of which are among the 

 best represented of the mammals. Development of the horse has 

 been relied on to a considerable extent as a guide to the various 

 Cenozoic horizons in terrestrial deposits. In the study of this group 

 much interest has been attached to the occasional discovery of inter- 

 vening types bridging previous hiatuses in the sequence. Plesippus 

 apparently bridges such a gap in the developmental sequence between 

 earlier Pliocene horses and Quaternary Equus. Plesippus shoshon- 

 ensis, the species encountered in the late Pliocene deposits near Hager- 

 man, Idaho, appears to be an advanced stage in this genus, bordering 

 on the equine types of recognized Pleistocene age. 



Attention of the United States National Museum was first directed 

 to the occurrence of fossil horse remains in the vicinity of Hagerman 

 in 1928 by Dr. H. T. Stearns, of the United States Geological Survey, 

 who was at the time engaged in a study of the geology and ground 

 water of the region. To Elmer Cook, a resident of Hagerman and 

 an amateur collector, belongs credit for discovering the fossils and 



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