As the subdivisions of the pseudomyid group of rats and mice are essential to the discussion of individual relationships, it is appropriate to emphasize that in 1910 Oldfield Thomas stressed their importance when defining the subgenera of Pseudomys (sensu lato) by stating that: "This genus contains species of very varied skull and molar structure, and it is with some hesitation that I leave such diverse species as, for example, P. australis and P. forresti under the same generic heading."
It is therefore clear that his subdivisions were not merely tentative, as suggested by Finlayson, but that it was actually their subgeneric status which Thomas considered possibly inadequate, when hesitating to retain them under the one genus. The elevation of the four subgenera to generic status in the Check-List by Iredale and Troughton is therefore in accordance with the views of Thomas, as confirmed by his fixation of the subgenera by the nomination of genotypes, comparative diagnoses, and grouping of their known forms.
A striking example of the value of generic distinction, between species of close external resemblance, is provided by the central Australian Gyomys desertor Troughton, 1932, which authors previously confused with the south Western Australian Thetomys nanus Gould, 1858. The cranial and dental features actually represent the extremes in group distinction, Thetomys having the strongly concave zygomatic plate and subsidiary cusp to m1 in contrast with the normal plate profile and absence of cusp in Gyomys. Etc.