When comparisons were made to ascertain specific and subspecific differences, only adults, or animals in which the enamel was worn through on the permanent P4 and p4 were used. Within this age range, only specimens in comparable pelage were used to ascertain differences in color. Miss Viola S. Schantz of the United States Fish and Wildlife Service, Mr. Alfred Bailey of the Colorado Museum of Natural History, Dr. W. H. Burt of the Museum of Zoology of the University of Michigan, Dr. Stephen D. Durrant of the Museum of Zoology of the University of Utah, Dr. Robert M. Stabler, curator of the Warren Collection of Colorado College, and Mr. James S. Findley, generously loaned specimens for my use. Doctors E. Raymond Hall, Rollin H. Baker, Robert W. Wilson, Keith R. Kelson, E. Lendell Cockrum, and other friends and associates have given valued suggestions and assistance. My wife, Alice M. White, made the illustrations and helped me record and analyze the data. Assistance with field work is acknowledged from the Kansas University Endowment Association, the National Science Foundation, and the United States Navy, Office of Naval Research, through contract No. NR161 791. ACCOUNTS OF SPECIES AND SUBSPECIES =Eutamias quadrivittatus= (Say) _Diagnosis._--Size medium; general tone of upper parts tawny; cranial breadth averaging between 16.0 and 16.8 mm.; baculum distinguishable from that of any other species by the combination of width of base less than 1/4 of length of shaft, shaft having a maximum diameter of more than 1/4 mm., and height of keel 1/4 of length of tip. =Eutamias quadrivittatus quadrivittatus= (Say) _Sciurus quadrivittatus_ Say, in Jones, Long's Expedition to Rocky Mountains, 2:45, 1823. _Eutamias quadrivittatus_, Miller and Rehn, Proc. Boston Soc. Nat. Hist., 30:43, December 27, 1901. _Tamias quadrivittatus gracilis_ J. A. Allen, Bull. Amer. Mus. Nat. Hist., 3:99, June 1890, Type from San Pedro, Santa Fe Co., New Mexico. _Eutamias quadrivittatus animosus_ Warren, Proc. Biol. Soc. Washington, 22:105, June 25, 1909. Type from Irwin Ranch, Las Animas County, Colorado. _Type._--None designated; from along Arkansas River, about 26 mi. below Canon City, Fremont County, Colorado; obtained on July 18, 1820. _Diagnosis._--Size medium; dorsal dark stripes blackish; sides Cinnamon to Clay Color; crown Light Drab; baculum large.