18. A new species of heteromyid rodent from the Middle Oligocene of northeastern Colorado with remarks on the skull. By Edwin C. Galbreath. Pp. 285-300, 2 plates. August 16, 1948. 19. Speciation in the Brazilian spiny rats (Genus Proechimys, Family Echimyidae). By João Moojen. Pp. 301-406, 140 figures in text. December 10, 1948. 20. Three new beavers from Utah. By Stephen D. Durrant and Harold S. Crane. Pp. 407-417, 7 figures in text. December 24, 1948. 21. Two new meadow mice from Michoacán, Mexico. By E. Raymond Hall. Pp. 423-427, 6 figures in text. December 24, 1948. 22. An annotated check list of the mammals of Michoacán, Mexico. By E. Raymond Hall and Bernardo Villa R. Pp. 431-472, 2 plates, 1 figure in text. December 27, 1949. 23. Subspeciation in the kangaroo rat, Dipodomys ordii. By Henry W. Setzer. Pp. 473-573, 27 figures in text, 7 tables. December 27, 1949. 24. Geographic range of the hooded skunk, Mephitis macroura, with description of a new subspecies from Mexico. By E. Raymond Hall and Walter W. Dalquest. Pp. 575-580, 1 figure in text. January 20, 1950. 25. Pipistrellus cinnamomeus Miller 1902 referred to the Genus Myotis. By E. Raymond Hall and Walter W. Dalquest. Pp. 581-590, 5 figures in text. January 20, 1950. 26. A synopsis of the American bats of the Genus Pipistrellus. By E. Raymond Hall and Walter W. Dalquest. Pp. 591-602, 1 figure in text. January 20, 1950. Index. Pp. 605-638. *Vol. 2. (Complete) Mammals of Washington. By Walter W. Dalquest. Pp. 1-444, 140 figures in text. April 9, 1948. Vol. 3. *1. The avifauna of Micronesia, its origin, evolution, and distribution. By Rollin H. Baker. Pp. 1-359, 16 figures in text. June 12, 1951. *2. A quantitative study of the nocturnal migration of birds. By George H. Lowery, Jr. Pp. 361-472, 47 figures in text. June 29, 1951. 3. Phylogeny of the waxwings and allied birds. By M. Dale Arvey. Pp. 473-530, 49 figures in text, 13 tables. October 10, 1951. 4. Birds from the state of Veracruz, Mexico. By George H. Lowery, Jr. and Walter W. Dalquest. Pp. 531-649, 7 figures in text, 2 tables. October 10, 1951. Index. Pp. 651-681. *Vol. 4. (Complete) American weasels. By E. Raymond Hall. Pp. 1-466, 41 plates, 31 figures in text. December 27, 1951.