southwestern shore of Hudson Bay, southern shore of Great Slave Lake and Yukon River, Yukon.

Genus Tamias Illiger

Tamias Illiger, J. K. W., Prodromus Syst. Mam. Avium, pp. 83, 1811. Type, Sciurus striatus Linnaeus.

Tamias, Howell, A. H., N. Amer. Fauna, 52:26, November 30, 1929.

Tamias, Ellerman, J. R., The families and genera of living rodents. British Mus. (Nat. Hist.), 1:426, June 8, 1940.

Tamias, Bryant, M. D., Amer. Midland Nat. 33:372, March, 1945.

Diagnosis.—Skull lightly built, narrow; postorbital process small and weak; lacrimal not elongated; infraorbital foramen lacks canal, relatively larger than in most sciurids; P3 absent; head of malleus elongated; plane of manubrium of malleus forms 60 degree angle with plane of lamina; hypohyal and ceratohyal bones of hyoid apparatus fused in adults; conjoining tendon of anterior and posterior digastric muscles rounded in cross section; keel on ventral surface of tip which curves upward in baculum; tail less than 38 per cent of total length; five longitudinal dark and four longitudinal light stripes present but two dorsal light stripes at least twice as broad as other stripes; four lateral dark stripes short.

Geographic range.—Eastern Nearctic. West to Turtle Mountains, North Dakota; eastern North Dakota, eastern South Dakota, Nebraska, Kansas, and Oklahoma. South to southern Louisiana, Mississippi, Alabama, northwestern Georgia. East to Atlantic Coast from South Carolina to Nova Scotia. North to northeastern Quebec and southern tip of Hudson Bay.

Discussion

Chipmunks are small striped squirrels that inhabit the Holarctic Realm and that are found in similar niches in each of the three regions: Palearctic, western Nearctic, and eastern Nearctic. Ellerman (1940) and Bryant (1945) placed the chipmunks in three subgenera, corresponding to the regions mentioned above, under the one genus Tamias. Critical examination of new and old evidence reveals, nevertheless, that the subgenera Eutamias and Neotamias of the genus Eutamias are more closely related to one another than either is to the genus Tamias. This relationship can be seen clearly in the structure of the malleus, baculum, hyoid apparatus, hyoid musculature, the presence or absence of P3, the projection of the anterior root of P4 in relation to the masseteric knob, and in the color pattern.

Because the genera Eutamias and Tamias occupy similar ecological niches, the structural similarities that permit these animals to be called chipmunks, show convergence, and thus can be assumed to be adaptive. These similarities are in the molars, in shape of the skull, in color pattern and in other features which have been used by many systematists to interpret the phylogenetic relationships of the squirrels. Pocock (1923:211), however, reviewed the taxonomic literature on sciurids and wrote: “The conclusion very forcibly suggested by the literature of the subject is the untrustworthiness of such characters.” Pocock (op. cit.), correctly in my opinion, then established a supraspecific classification of the sciurids based almost exclusively on the structure of the baculum and glans penis. I have studied the baculum in chipmunks and in all the major supraspecific groups 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 zoographicis utriuque classis, pp. xviii + 302, C. Salfeld.

Laxmann, M. E.
1769. Sibiriche Brief, pp. iv + 104, Gottingen and Gotha.

Layne, J. N.
1952. The os genitale of the red squirrel, Tamiasciurus. Jour. Mamm. 33:457-459, 1 fig., November 19.

Linnaeus, C.
1758. Systema Naturae. 1:1-824.

Merriam, C. H.
1897. Notes on the chipmunks of the genus Eutamias occurring west of the east base of the Cascade-Sierra system, with descriptions of new forms. Proc. Biol. Soc. Washington, 11:189-212, July 1.

Pocock, R. I.
1923. The classification of the Sciuridae. Proc. Zool. Soc. London, 1923:209-246, June.

Say, T. (in Edwin James).
1823. Account of an expedition from Pittsburgh to the Rocky Mountains, performed in the years 1819 and 1820, under the command of Major Stephen H. Long. From the notes of Major Long, Mr. T. Say, and other gentlemen of the exploring party. Compiled