Carnegie Museum, J. K. Doutt. California Academy of Sciences, Robert T. Orr. Chicago Natural History Museum, Phillip H. Hershkovitz. Cleveland Museum of Natural History (Collection now a part of Museum of Zoology, University of Michigan, W. H. Burt, E. T. Hooper). Louisiana State University, Museum of Natural History, George H. Lowery, Jr. Los Angeles County Museum, Charles A. McLaughlin. United States National Museum (Biological Survey Collections), David A. Johnson, and Viola S. Schantz. United States National Museum, Division of Vertebrate Paleontology, C. Lewis Gazin. University of Arizona, E. L. Cockrum, and G. VR. Bradshaw. University of California, Museum of Vertebrate Zoology, Seth B. Benson, and W. Z. Lidicker. University of Illinois, Museum of Natural History, Donald F. Hoffmeister. University of Michigan, Museum of Zoology, W. H. Burt, E. T. Hooper, and Claude W. Hibbard. University of New Mexico, James S. Findley. University of Texas, Frank W. Blair. Texas A & M, Cooperative Wildlife Research Collection, W. B. Davis. The Museum, Michigan State University, Rollin H. Baker. University of Florida Collections, James N. Layne. I am especially grateful to Professor E. Raymond Hall who guided me in my study and gave critical assistance with the manuscript. Additional appreciated suggestions were made by Professors A. Byron Leonard, Robert W. Wilson, Henry S. Fitch, Ronald L. McGregor, and fellow graduate students. For the illustrations, I am indebted to Mrs. Lorna Cordonnier, Miss Lucy Remple and Mrs. Connie Spitz. Mr. B. J. Wilks of the University of Texas, Department of Zoology, provided a number of living pygmy mice for study in captivity. Mr. J. Raymond Alcorn and his son, Albert, collected a large share of specimens of pygmy mice now in the University of Kansas, Museum of Natural History. My wife, Patricia, aided me in secretarial work and typing of the manuscript. For financial assistance, I am indebted to the National Science Foundation when I was a Research Assistant, to the Sigma Xi-RESA Research Fund for a Grant-in-Aid, and to the Kansas University Endowment Association through its A. Henley Aid Fund, and the Watkins Fund for out-of-state field work by the Museum of Natural History. PALEONTOLOGY OF THE GENUS Five fossil species, all extinct, have been assigned to the genus and range in time from early late Pliocene (Saw Rock Canyon fauna of Hibbard, 1953:408) to Mid-Pleistocene (see Hibbard, 1958:25, who assigns the Curtis Ranch fauna to late Kansan or early Yarmouth). I examined all known fossil material and compared it with Recent material. When the antiquity of the genus is considered, the degree of difference between the oldest fossil species and the two living species is much less than might be expected. =Baiomys sawrockensis= Hibbard