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. Examination of series of mallei of _Eutamias_ and _Tamias_ indicate that there is slight individual variation, slight variation with age, and no secondary sexual variation. Intraspecific variation in the subgenus _Neotamias_ is slight, consisting of differences in size. Specimens of the subgenus _Eutamias_ from Manchuria have mallei which are morphologically close to the mallei of the subgenus _Neotamias_. [Illustration: FIGS. 1-3. Dorsomedial views of left malleus. FIG. 1. _Tamias striatus lysteri_, No. 11920 sex?; from Carroll Co., New Hampshire. FIG. 2. _Eutamias sibiricus asiaticus_, No. 199637 male NM; from I-mien-po, N. Kirin, Manchuria. FIG. 3. _Eutamias townsendii senex_, No. 165 male; from Lake Tahoe, California.] _Structure of the Baculum._--In discussing the baculum in _Eutamias_ and _Tamias_, it seems desirable to do so in the light of the structure of the baculum in other sciurids. The bacula of North American sciurids are divisible into six distinct types represented by those of the genera _Spermophilus_, _Marmota_, _Sciurus_, _Tamiasciurus_, _Eutamias_, and _Glaucomys_. The type of baculum in _Spermophilus_ is spoonshaped with a ventral process that is spinelike or keellike. Also, spines usually are present along the margin of the "spoon." The base (proximal end) of the baculum is broad, and some species have a winglike process extending dorsally and partly covering a longitudinal groove. The shaft is more or less curved downward in the middle (see figs. 7, 10).