Many people in Michoacán favored the field parties with quarters, transportation, and valuable information, which greatly facilitated the field work. In this respect I am especially indebted to Ingeniero Ruben Erbina of Ingenieros Civiles Asociados, who not only let us use his home as our headquarters, but through a letter of introduction gave us the "key" to southern Michoacán. Ingeniero Pedro Tonda aided us in Arteaga and San Salvador. Ingeniero Anastacio Peréz Alfaro of the Comisión Tepalcatepec in Uruapan provided the latest maps of southern Michoacán and much essential information pertaining to travel conditions in the area. Señor Nefty Mendoza gave us a home in Dos Aguas; this kindness allowed us to work in this interesting region during the height of the rainy season. Mr. and Mrs. Bob Thomas let us make use of their facilities at Hacienda Zirimícuaro. The naval officers at the Estación Marina at Playa Azul made pleasant what might have been a dreadful stay in that small coastal village. To the managers and pilots of Lineas Aereas Picho in Uruapan I owe special thanks for going out of their way on more than one occasion to transport a stranded snake-hunter. Throughout the months of field work beginning in 1955 I constantly have been aided by the authorities and workers of the Comisión Tepalcatepec, a subdivision of the Secretaria de Caminos y Obras Publicas, and of the private corporation, Ingenieros Civiles Asociados. Much of the field work in Michoacán was made possible only through the co-operation of the natives who supplied mules, acted as guides, and aided in the collection of specimens. I have learned a great deal from these people. They will never see this report. Their work as guides, muleteers, and collectors greatly assisted me with the mountains of equipment that had to be piled on the backs of scrawny mules for transportation to places where the natives seldom trod. Their efforts in behalf of Don Guillermo never will be forgotten; I extend an especially hearty _muchas gracias_ to Benjamin, Ignacio, Jesús, Lorenzo, Mariano, and Remigio. Much of the work on this report was done while I was associated with the Museum of Zoology at the University of Michigan. I thank Norman E. Hartweg and T. H. Hubbell for making available to me the facilities of the museum and for their numerous courtesies that aided me so much. My field work in Michoacán was supported by the Museum of Zoology at the University of Michigan (1951), by the Horace H. Rackham School of Graduate Studies of the University of Michigan (1955), by the Penrose Fund of the American Philosophical Society (1956), by the Bache Fund of the National Academy of Sciences (1958), and by the University of Kansas Endowment Association (1960). Permits for collecting specimens in México were provided by the Dirección General de Caza through the courtesy of Ing. Juan Lozano Franco and Luis Macías Arellano. Historical Account