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 11'. Tip less than 28 per cent of length of shaft. _Eutamias minimus_, p. 617 7'. Base widened and dorsoventrally thickened. _Eutamias townsendii_, p. 618 5'. Shaft thick; shaft more than .25 mm. in diameter at widest point. 12. Length of shaft less than 3.00 mm.; length of tip less than 1.10 mm. _Eutamias quadrivittatus hopiensis_, p. 622 12'. Length of shaft more than 3.10 mm.; length of tip more than 1.15 mm. 13. Tip less than 28 per cent of length of shaft. _Eutamias quadrimaculatus_, p. 624 13'. Tip more than 29 per cent of length of shaft. 14. Angle formed by tip and shaft more than 140°; ridges on either side of tip indistinct. _Eutamias cinereicollis_, p. 624 14'. Angle formed by tip and shaft less than 135°; ridges on either side of tip distinct. 15. Shaft less than 3.65 mm. in length, and .55 mm. or less in diameter at widest point. _Eutamias quadrivittatus quadrivittatus_, p. 621 15'. Shaft usually more than 3.65 mm. in length, but when shorter, diameter is .60 mm. or more at widest point.