1936. =perotae= (_Mustela frenata_) Hall, Carnegie Inst. Washington, publ. no. 473, p. 100, November 20, 1936, applies to long-tailed weasel of the mountains along the Puebla-México boundary. 1938. =boliviensis= (_Mustela frenata_) Hall, Proc. Biol. Soc. Washington, 51:67, May 18, 1938, applies to the southernmost known long-tailed weasel which is in the Lake Titicaca region in Perú and Bolivia. 1944. =salva= (_Mustela erminea_) Hall, Proc. Biol. Soc. Washington, 57:35, June 28, 1944, applies to the ermine of Admiralty Island, southeastern Alaska. 1944. =initis= (_Mustela erminea_) Hall, Proc. Biol. Soc. Washington, 57:37, June 28, 1944, applies to the ermine of Baranof and Chichagof islands, southeastern Alaska. 1944. =celenda= (_Mustela erminea_) Hall, Proc. Biol. Soc. Washington, 57:38, June 28, 1944, applies to the ermine of Prince of Wales, Dall and Long islands, Alaska. 1944. =seclusa= (_Mustela erminea_) Hall, Proc. Biol. Soc. Washington, 57:39, June 28, 1944, applies to the ermine of Suemez Island, southeastern Alaska. 1945. =invicta= (_Mustela erminea_) Hall, Jour. Mamm., 26:75, February 27, 1945, applies to the ermine of the Rocky Mountains for several hundred miles both north and south of the United States-Canadian boundary. 1945. =fallenda= (_Mustela erminea_) Hall, Jour. Mamm., 26:79, February 27, 1945, applies to the ermine of the coastal mainland in southern British Columbia and northern Washington. 1945. =olympica= (_Mustela erminea_) Hall, Jour. Mamm., 26:81, February 27, 1945, applies to the diminutive ermine of the Olympic Peninsula, state of Washington. 1945. =gulosa= (_Mustela erminea_) Hall, Jour. Mamm., 26:84, February 27, 1945, applies to the diminutive ermine of the Cascades in Washington. 1945. =bangsi= (_Mustela erminea_) Hall, Jour. Mamm., 26:176, July 19, 1945, is the name applied today to the ermine of the western Great Lakes region. In 1925 when this study was begun, the American weasels (subgenus _Mustela_ proper) were arranged as belonging to 47 kinds (including subspecies) of 29 full species. In the present account a total of 68 kinds, belonging to 4 full species are recognized in the subgenus _Mustela_. The increase in number of subspecies and the decrease in number of species are the nomenclatural results ordinarily obtained in this decade from a systematic study of a genus of American mammals. CHECK-LIST OF AMERICAN SPECIES AND SUBSPECIES OF THE GENUS MUSTELA Subgenus =MUSTELA= Linnaeus