of
Nearctic squirrels. The bacula of the Nearctic squirrels and those
of the Palearctic and Indian squirrels, other than the chipmunks, are
described and figured by Pocock (_op. cit._).

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

CATESBY, M.
1743. The natural history of Carolina, Florida, and the Bahama Islands,
etc., 2:i-xliv, 1-20, 1-100.

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.

GMELIN, J. F.
1788. Systema Naturae. 1:1-500.

HOWELL, A. H.
1929. Revision of the American chipmunks (genera _Tamias_ and _Eutamias_).
N. Amer. Fauna 52:1-157, 10 pls., 9 maps, November 30.

ILLIGER, J. K. W.
1811. Prodromus systematis mammalium et avium additis terminis