Hollister (1919:40) listed eight species of Tachyorytces, noted that "all have constant characters of differentiation, and intergradation between any two of them is not indicated by this material," but speculated that the "numerous forms will doubtlessly be connected by complete chains of intergrades and the final monographer of the genus will be obliged to reduce many of the named forms to the rank of subspecies." G. M. Allen (1939) and Ellerman (1941) listed 14 species, but these were reduced to T. macrocephalus and T. splendens by Misonne (1974), an arrangement followed by Corbet and Hill (1991). Of the twenty forms described, Rahm (1980) considered about 16 of them to be valid subspecies of T. splendens.

The abrupt reduction from 14 to 2 species was not based on reevaluation of morphological differences that characterize the named forms. Aside from Bekele’s (1986) inconclusive univariate analyses of craniometric data from samples of Tachyoryctes, no assessment of morphological variation in the genus is available in context of a systematic revision. Until such a study provides documentation and definition of specific limits and geographic distributions, we return to a modified treatment of the arrangement by G. M. Allen (1939) and Ellerman (1941), which reflects the diagnostic information now available from specimen study and provides a working estimate of biological reality. Morphology and morphometry of the lungs of a Kenyan sample of Tachyoryctes contrasted with Heterocephalus by Maina et al. (1992), stomach morphology described by Rahm (1976), morphogenesis of fetal membranes and placenta by Makori et al. (1991), and lower incisor shape compared with other muroids by Millien-Parra (2000a).