AMNH--American Museum of Natural History. BYU--Brigham Young University. CNHM--Chicago Natural History Museum. CM--Carnegie Museum. CMNH--Colorado Museum of Natural History. DJC--Dixie Junior College. DRD--Donald R. Dickey Collection. KU--Museum of Natural History, University of Kansas. LACM--Los Angeles County Museum. MHS--Collection of Myron H. Swenk. MVZ--Museum of Vertebrate Zoology, University of California. OU--Museum of Zoology, University of Oklahoma. RH--Collection of Ross Hardy. UM--Museum of Zoology, University of Michigan. UN--Museum of Natural History, University of Nebraska. USAC--Utah State Agricultural College. USBS--United States Biological Surveys Collection. USNM--United States National Museum. UU--Museum of Zoology, University of Utah. TCWC--Texas Coöperative Wildlife Collection. This study is based on 3,732 specimens which were assembled at the Museum of Natural History, University of Kansas, or studied at other institutions. For the loan of this material and for the opportunity afforded for its study, I am extremely grateful to the authorities of each of these institutions and to the owners of the private collections. Acknowledgement is made to the Office of Research and Inventions of the United States Navy for assistance with the field work which permitted the acquisition of essential specimens from several of the critical geographic areas while the author was research assistant on a larger over-all project (N6 ori-164-T02) of which the determination of the geographic range of this rodent species, a potential host of tularemia, was one facet. Tularemia was not detected in this genus. I extend my thanks also to Professor Stephen D. Durrant, of the University of Utah, for helpful corrections in the preparation of the manuscript; to Mrs. Virginia Cassell Unruh for the preparation of the drawings; to Professor E. Raymond Hall, of the University of Kansas, for guidance in the study and critical assistance with the manuscript; to Professors H. H. Lane and Worthie H. Horr for valued suggestions; to Mr. J. R. Alcorn for providing specimens for dissection when he was working under the University of Kansas endowment fund; and to the other friends and associates who have given of their time and criticism. PALEONTOLOGY