Part 4

A variation that does not readily fall in any one of the three categories mentioned above is the degree of development of the lambdoidal crest. The crest is least developed in the subgenus Neotamias and most developed in the genus Tamias. The larger the skull, the more the lambdoidal crest is developed; seemingly, therefore, the degree of development is an expression of size of the skull and may be determined by heterogonic growth.

DISCUSSION

As shown in table 1, there are ten characters by means of which Eutamias and Tamias can be separated consistently. The subgenus Eutamias occurs on the Asiatic side and the subgenus Neotamias occurs on the North American side of Bering Strait, yet the two subgenera agree in the ten features referred to. Although the subgenus Neotamias and the genus Tamias occur together in parts of the United States and Canada, they differ in the ten features, indicating that the subgenera Eutamias and Neotamias are more closely related to each other than either is to Tamias.

TABLE 1.--CHARACTERS BY MEANS OF WHICH THE GENERA EUTAMIAS AND TAMIAS CAN BE DISTINGUISHED

======================================================================= Character | Eutamias | Tamias -------------------------------------+----------------+---------------- Shape of head of malleus. | not elongated. | elongated. -------------------------------------+----------------+---------------- Angle formed by planes of lamina and | approximately | approximately manubrium of malleus. | 90 degrees. | 60 degrees. -------------------------------------+----------------+---------------- Position of keel on tip of baculum. | dorsal. | ventral. -------------------------------------+----------------+---------------- Relation of hypohyal and ceratohyal | fused in | never fused. bones of hyoid apparatus. | adults. | -------------------------------------+----------------+---------------- Appearance in cross section of | flattened. | rounded. conjoining tendon of anterior and | | posterior digastric muscles. | | -------------------------------------+----------------+---------------- Presence or absence of P3. | present. | absent. -------------------------------------+----------------+---------------- Projection of anterior root of P4 in | buccal. | lingual. relation to masseteric knob. | | -------------------------------------+----------------+---------------- Length of tail in relation to total | more than | less than length. | 40 per cent. | 38 per cent. -------------------------------------+----------------+---------------- Width of longitudinal stripes. | subequal. | median pair of | | light stripes | | twice as wide | | as others. -------------------------------------+----------------+---------------- Length of lateral longitudinal light | outermost | both pairs stripes. | pair short. | short. -----------------------------------------------------------------------

It must be pointed out here that the subgenus Neotamias always differs from both the subgenus Eutamias and the genus Tamias in pointed versus rounded pinna of ear (see table 2) and in the supraorbital notch being slightly posterior to or even with, instead of distinctly anterior to, the posterior notch of the zygomatic plate. The relative position of these two notches, however, seems to be a matter of relative (heterogonic) growth. Further, the base of the postorbital process of the frontal usually is narrower (relative to the length of the process) in the subgenus Neotamias but there is a gradation in this feature in Neotamias culminating in the species E. townsendii in which the bases of the processes are relatively as broad as in the subgenus Eutamias and the genus Tamias. The same condition obtains in the shape of the infraorbital foramen which is subovate to rounded in the subgenus Neotamias and always rounded in the other chipmunks.

TABLE 2.--CHARACTERS BY MEANS OF WHICH THE SUBGENUS EUTAMIAS AND THE GENUS TAMIAS MAY BE DISTINGUISHED FROM THE SUBGENUS NEOTAMIAS

======================================================================== Character | subgenus | subgenus | genus | Neotamias | Eutamias | Tamias -------------------------------+--------------+-------------+----------- Shape of infraorbital foramen. | subovate to | always | always | rounded. | rounded. | rounded. -------------------------------+--------------+-------------+----------- Relative width of the | narrow to | | postorbital process at base. | broad. | broad. | broad. -------------------------------+--------------+-------------+----------- Position of supraorbital notch | even with or | | in relation to posterior | slightly | anterior. | anterior. notch of zygomatic plate. | posterior. | | -------------------------------+--------------+-------------+----------- Convergence, posteriorly, | not always. | not always. | always. of upper tooth-rows. | | | -------------------------------+--------------+-------------+----------- Degree of constriction of | slight | marked. | marked. interorbital region. | to marked. | | -------------------------------+--------------+-------------+----------- Shape of pinna. | long and | broad and | broad and | pointed. | rounded. | rounded. ------------------------------------------------------------------------

These differences of Neotamias are so slight in comparison with the similarities (ten features mentioned above) that Neotamias here is accorded only subgeneric rank under the genus Eutamias, instead of generic rank.

Howell's (1929) arrangement of the genera and subgenera of chipmunks is judged to be correct as indicated by the following arrangement that I propose.

GENERA AND SUBGENERA

Genus #Eutamias# Trouessart

Eutamias Trouessart, E. L. Catal. Mamm. viv. et foss., Rodentia, in Bull. Soc. d'Etudes Sci. d'Angers, 10:86-87, 1880. Type Sciurus striatus asiaticus Gmelin.

Eutamias, Merriam, C. H., Proc. Biol. Soc. Washington, 11:189-190, July 1, 1897.

Eutamias, 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., 33:732, March 1945.

Diagnosis.--Skull lightly built, narrow; postorbital process light and weak; lacrimal not elongated; infraorbital foramen lacks canal, relatively larger than in most sciurids; P3 present; head of malleus not elongated; plane of manubrium of malleus 90 degrees to plane of lamina; hypohyal and ceratohyal bones of hyoid apparatus fused in adults; conjoining tendon between anterior and posterior sets of digastric muscles ribbonlike; keel on dorsal side of tip of baculum; tail more than 40 per cent of total length; five longitudinal dark stripes evenly spaced and subequal in width; two lateral dark stripes short.

Subgenus #Eutamias# Trouessart

Eutamias Trouessart, E. L. Catal. Mamm. viv. et foss., Rodentia, in Bull. Soc. d'Etudes Sci. d'Angers 10:86-87, 1880. Type Sciurus striatus asiaticus Gmelin.

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

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

Eutamias, Bryant, M. D., Amer. Midland Nat. 33:732, March 1945.

Diagnosis.--Size large; lambdoidal crest moderately developed; supraorbital notches distinctly anterior to posterior notch of zygomatic plate; baculum with faint keel on dorsal surface of tip which curves upward; pelage coarse; ears broad, rounded, of medium height.

Geographic range.--Palearctic. West to Dvina and Kama rivers, Vologda, and Kazan, in European Russia. South to southern Ural Mountains, Altai Mountains; Kansu, Szechwan, Shensi, Shansi, and Chihli provinces of China; Manchuria and Korea. East to Hokkaido Island, Japan; Kunashiri Island, southern Kurile Islands; Sakhalin Island, and Yakutsk, Siberia. North nearly to Arctic Coast in Siberia and European Russia (Ellerman and Morrison-Scott 1951:503).

Subgenus #Neotamias# Howell

Neotamias Howell, A. H., N. Amer. Fauna, 52:26, November 30, 1929. Type, Eutamias merriami J. A. Allen [=Tamias asiaticus merriami J. A. Allen].

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