All specimens of which measurements are here recorded, as for example in the tables, are fully adult; each specimen shows some wear on each of the four upper molariform teeth unless otherwise indicated. Capitalized color terms are after Ridgway "Color Standards and Color Nomenclature," Washington, D. C., U. S. A., 1912. One setiform was taken from the animal and placed over the rectangles in Ridgway's charts and the examination made under a microscope with low (×7) magnification and natural light. This method was found to give the most satisfactory results. The following abbreviations are used for names of institutions: AMNH--American Museum of Natural History. CNHM--Chicago Natural History Museum. DZ--Departamento de Zoologia da Secretaria de Agricultura, São Paulo, Brazil. MCZ--Museum of Comparative Zoology at Harvard College. MN--Museu Nacional, Brazil. MZ--Museum of Zoology, University of Michigan. SEPFA--Serviço de Estudos e Pesquisas sobre a Febre Amarela, Brazil. USNM--United States National Museum. UZM--Universitets Zoologiske Museum, Copenhagen. ACKNOWLEDGMENTS Approximately two thousand skins and skulls were assembled at the Museum of Natural History, University of Kansas, through the coöperation of the authorities in the various institutions of North America, Brazil and Denmark, as listed immediately above. This comprehensive material was used to obtain a more complete understanding of the group, and for the loan of these specimens I am extremely grateful to the authorities of each of the institutions. First of all I acknowledge the encouragement given me in the _Proechimys_ project by Heloisa Alberto Torres, Director of the Museu Nacional, Rio de Janeiro. I extend my thanks also to 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 of the skulls; to Miss Alice M. Bruce for assistance in drawing the maps; and to my daughter, Julieta, for help in assembling data and for typing. Dr. Remington Kellogg, Curator of Mammals in the United States National Museum, and the late Dr. Wilfred H. Osgood, formerly Curator Emeritus of the Department of Zoology in the Chicago Natural History Museum, generously permitted me to use their private lists of South American mammals. These lists contain much unpublished data, as for example, proof, in Kellogg's list, that _Proechimys guyannensis_ (E. Geoffroy Saint-Hilaire, 1803) antedates _P. cayennensis_ (Desmarest, 1817). I register here my gratitude to both these zoölogists and acknowledge other critical assistance from Dr. Kellogg.