Chromosome Numbers and Systematic Relations in the Genus Thomomys (Rodentia: Geomyidae)

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Abstract
Published data concerning chromosome numbers of forms within the genus Thomomys are reviewed and additional data are presented. These indicate that the living species of the genus can be arranged in two groups, one with diploid chromosome numbers in the range of 40 to 60 and a second in the range of 74 to 82. Other morphological characters support this dichotomy, suggesting that each group is composed of closely related species. These relationships are best expressed by grouping the nine living and three extinct species into two subgenera: Thomomys and Megascapheus. A third subgenus, Plesiothomomys, includes three additional extinct species.