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. NR 161 791. VARIATION _Individual variation._--Individual variation is small. This is shown by a coefficient of variability of only 3.85 in the length of the shaft in a series of 12 specimens of _E. umbrinus umbrinus_ from Paradise Park, 21 mi. W and 15 mi. N Vernal, 10,050 ft., Uintah County, Utah. _Variation with age._--In the chipmunks the baculum varies but little with age. In the youngest specimens that I have taken, the M3 and m3 have not yet erupted and there is no wear on P4 and p4; nevertheless, the baculum in these specimens has nearly an adult configuration and size. In juvenal _Eutamias minimus_ the tip of the baculum is longer in relation to the length of the shaft than it is in adults; the tip is 18 to 28 per cent of the length of the shaft in adults, as opposed to 29 to 32 per cent in juveniles. _Aberrations._--In a small percentage of specimens of _E. minimus_ and _E. umbrinus_ the baculum is small and S-shaped, even in adults. _Variations of taxonomic worth._--Variations in this category are described in the section immediately following the key. KEY TO THE BACULA IN EUTAMIAS OF WESTERN NORTH AMERICA 1. Distal 1/2 to 2/3 of shaft markedly compressed laterally; base markedly widened. 2. Distal 1/2 of shaft laterally compressed and curved downward to base of tip. 3. Height of keel 1/2 of length of tip; keel markedly enlarged. _Eutamias bulleri_, p. 627 3'. Height of keel 1/4 of length of tip; keel not markedly enlarged. _Eutamias umbrinus_ and _E. palmeri_, pp. 626, 627 2'. Distal 2/3 of shaft laterally compressed and curved downward to base of tip. 4. Base of keel 1/3 of length of tip; angle formed by tip and shaft less than 100° _Eutamias speciosus_, p. 625 4'. Base of keel 1/2 of length of tip; angle formed by tip and shaft more than 102° _Eutamias panamintinus_, p. 625 1'. Distal 1/12 to 2/5 of shaft slightly compressed laterally; base not markedly widened. 5. Shaft thin; shaft less than .20 mm. in diameter at widest point. 6. Ridges on either side of keel enlarged, partially obscuring lateral view of keel; height of keel 1/10 of length of tip. _Eutamias sonomae_, p. 619 6'. Ridges on either side of tip not enlarged, not partially obscuring lateral view of keel; height of keel at least 1/7 of length of tip. 7. Base not widened or dorsoventrally thickened.