Measurements, unless otherwise noted, are of adults and are given in millimeters. External measurements are in the following order: total length; length of tail vertebrae; length of hind foot; length of ear from notch. Capitalized color terms are those of Ridgway, Color Standards and Color Nomenclature, Washington, D. C., 1912. Capital letters designate teeth in the upper jaws and lower case letters designate teeth in the lower jaws; for example, M2 refers to the second upper molar and m2 refers to the second lower molar. The localities of specimens examined and additional records are listed from north to south and their geographic positions can be found in the gazetteer and on the map (Fig. 4). Most of the specimens examined are in the Museum of Natural History of the University of Kansas. Unless otherwise indicated, catalogue numbers relate to that collection. A few specimens from other collections were seen. Abbreviations identifying those collections are: UMMZ, the University of Michigan Museum of Zoology; AMNH, the American Museum of Natural History; and GMS, George M. Sutton collection (University of Oklahoma). I am grateful to Prof. E. Raymond Hall and Dr. J. Knox Jones, Jr., for their advice and kind help that have enabled me to complete this work. I thank Dr. William E. Duellman for his advice concerning Zoogeography and Biologist Gastón Guzmán for help with the names of plants. For the loan of specimens I am grateful to Dr. George M. Sutton of the University of Oklahoma, to Dr. David H. Johnson and Dr. Richard H. Manville of the United States National Museum, to Drs. William H. Burt and Emmet T. Hooper of the University of Michigan Museum of Zoology, and to Dr. Richard Van Gelder of the American Museum of Natural History. I thank, also, Dr. William Z. Lidicker, Jr., for information about the locality called Lulú, and the collectors from the Museum of Natural History, especially Gerd H. Heinrich, William J. Schaldach, Percy L. Clifton, and John H. Bodley. I am grateful also to Charles A. Long and to several other persons, not named here, who helped me in some way to complete my study of the mammals of Tamaulipas. Most of the field work was financed by the Kansas University Endowment Association. Some laboratory work was done when the author was half-time Research Assistant under Grant No. 56 G 103 from the National Science Foundation. GAZETTEER The specimens examined and additional records are listed with reference to the following place names. The geographic position of each was taken from the maps of the American Geographical Society of New York, scale 1:1,000,000, and the Atlas Geográfico de la República Mexicana, scale 1:500,000.