_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.
8. Shaft more than 4.5 mm. in length; tip 16 per cent of length of shaft; shaft strongly incised on dorsal side of base _Eutamias merriami_, p. 621
8'. Shaft less than 4.4 mm. in length; tip more than 25 per cent of length of shaft; shaft not incised on dorsal side of base.
9. Height of keel 1/7 of length of tip; angle formed by tip and shaft distinct _Eutamias alpinus_, p. 616
9'. Height of keel at least 1/5 of length of tip; angle formed by tip and shaft poorly defined.
10. Height of keel 1/3 of length of tip; angle formed by tip and shaft 140° _Eutamias dorsalis_, p. 620
10'. Height of keel 1/5 of length of tip; angle formed by tip and shaft 130° or less.
11. Tip more than 29 per cent of length of shaft. _Eutamias amoenus_, p. 619