^ 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 48610— 3G 1 281
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