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.
16. Diameter of shaft at widest point less than .58 mm.; tip less than 35 per cent of length of shaft. _Eutamias ruficaudus ruficaudus_, p. 622
16'. Diameter of shaft at widest point more than .65 mm.; tip more than 40 per cent of length of shaft. _Eutamias ruficaudus simulans_, p. 623
Pelage silky; tail bright orange beneath; markings relatively obscure; size small; skull broad, flattened, and large in proportion to body.
Baculum: Shaft thin; keel low, 1/7 of length of tip; tip 39 per cent of length of shaft; angle formed by tip and shaft 135°; distal 1/3 of shaft slightly compressed laterally; base slightly wider than shaft; shaft short, 2.17 mm.
Differs from _E. speciosus_, _E. panamintinus_, _E. umbrinus_, _E. palmeri_, and _E. bulleri_, in base not markedly widened, and shaft thinner; from _E. quadrivittatus_, _E. ruficaudus_, _E. cinereicollis_, and _E. quadrimaculatus_, in shaft thinner, baculum shorter; from _E. townsendii_, in base not dorsoventrally thickened, base not so widened; from _E. sonomae_, in ridges on either side of tip not enlarged, base not dorsoventrally thickened; from _E. amoenus_, _E. minimus_, and _E. dorsalis_, in keel lower, angle formed by tip and shaft more distinct; from _E. merriami_, in baculum markedly shorter, base not incised dorsally.