The baculum in Eutamias, in general plan of structure, resembles the baculum in the genera Callosciurus, Menetes, Rhinosciurus, Lariscus, Dremomys, and Nannosciurus, of the tribe Callosciurini Simpson. The baculum in Tamias, in general plan of structure, resembles that in Spermophilus (=Citellus) and Cynomys of the tribe Marmotini Simpson. These tribes, designated by Simpson (1945:79), are based on the corresponding subfamilies defined by Pocock (1923:239-240) primarily on differences in the structure of the baculum. I assign Tamias to the tribe Marmotini. I assign Eutamias to the tribe Callosciurini, but do so only tentatively because I have not, at first hand, studied the bacula of most of the Callosciurini. The fossil record is too incomplete to reveal the time when the two tribes diverged. The subgenera Eutamias and Neotamias are closely related. Indications are that the divergence of the two subgenera occurred, geologically, but a short time ago, possibly in Pleistocene time.
Conclusions
1. Eutamias and Tamias are distinct genera of chipmunks.
2. The subgenera Eutamias and Neotamias are valid, for, Eutamias sibiricus differs from all the species of the subgenus Neotamias to a greater degree than these species differ from one another.
3. The genera Eutamias and Tamias probably evolved from two distinct lines of sciurids; one line (Eutamias) is represented by the tribe Callosciurini, and the other (Tamias) by the tribe Marmotini.
Literature Cited
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Ellerman, J. R.
1940. The families and genera of living rodents. British Mus. (Nat. Hist.), Vol. 1, pp. xxvi + 689, 189 figs., June 8.
Ellerman, J. R., and T. C. S. Morrison-Scott.
1951. Checklist of Palearctic and Indian mammals. British Mus. (Nat. Hist.), pp. 1-810, 1 map, November 30.