Characters previously mentioned in the literature as having taxonomic worth for supraspecific categories of chipmunks were checked by me on specimens old enough to have worn permanent premolars. Some structural features not previously used were found to have taxonomic significance. The baculum in each of the supraspecific categories of sciurids of North America was examined; the bacula were processed by the method described by White (1951:125) to obviate “variation” caused by shriveling of the smaller bacula or breaking of the more delicate parts of the larger bacula. Mallei and hyoid bones of the genera and subgenera of the chipmunks were mostly studied in the dry state. Part of the hyoid musculature in these same groups of chipmunks was dissected.
In all, I studied more than 1,000 skulls and skins of the subgenus Neotamias, approximately 50 skulls and skins of Tamias striatus, and 15 skulls and skins of the subgenus Eutamias (Eutamias sibiricus asiaticus from Manchuria). Numerous other specimens were examined but not in such detail.
I am grateful to Professor E. Raymond Hall for guidance in the study. For encouragement and advice I am grateful also to Doctors Robert W. Wilson, Cecil G. Lalicker, Edwin C. Galbreath, Keith R. Kelson, E. Lendell Cockrum, Olin L. Webb, and others at the Museum of Natural History, and in the Department of Zoology of the University of Kansas. My wife, Alice M. White, made the drawings and helped me in many other ways. For lending specimens I thank Dr. David H. Johnson of the United States National Museum, and Dr. George C. Rinker of the Department of Anatomy, University of Michigan.
Assistance with field work is acknowledged from the Kansas University Endowment Association, the National Science Foundation, and the United States Navy, Office of Naval Research, through contract No. NR161 791.
Evaluation of Characters
The following paragraphs treat the characters listed by Howell, Ellerman, and Bryant, and such additional characters as I have found useful in characterizing the genera and subgenera of chipmunks. Some of the findings, I think, illustrate how study of such mammalian structures as the baculum, malleus, and hyoid apparatus—structures that seem to be little influenced by the changing external environment—clarifies relationships, if these previously were estimated only from other parts of the anatomy of Recent specimens.
The structural features and characters to be discussed, or listed, below may be arranged in three categories as follows: 1) Characters in which the subgenera Eutamias and Neotamias agree but are different from the genus Tamias; 2) Characters in which the subgenus Eutamias and the genus Tamias agree but are different from the subgenus Neotamias; 3) Structural features that are too weakly expressed to be of taxonomic use.
Characters in which the Subgenera Eutamias and Neotamias Agree, but Differ from the Genus Tamias
Structure of the Malleus.—The malleus in chipmunks is composed of a head and neck, a manubrium which has a spatulate process at the end opposite the head, and a muscular process situated about halfway between the spatulate process and the head of the malleus. An articular facet begins on the manubrium near the neck and spirals halfway around the head of the malleus. A lamina extends from the anterior edge of the head and neck, tapers to a point and joins the tympanic bulla anteriorly where there is a suture between the lamina and bulla. The lamina is one half as long as the rest of the malleus (see figs. [1-3]).
The head of the malleus in Tamias is clearly more elongated than in Eutamias. The plane formed by the lamina in Eutamias makes an angle of approximately 90 degrees with the plane formed by the manubrium; in Tamias the two planes make an angle of approximately 60 degrees.