From _arctica_, _polaris_, _semplei_ and _kadiacensis_, _richardsonii_ differs in: Color darker; ventral side of tail same color as upper parts; light-colored underparts a fifth narrower; black tip of tail by actual measurement a fifth shorter and averaging less than two-thirds rather than more than four-fifths of length of tail-vertebrae. From _cicognanii_, _richardsonii_ differs in that the underparts are a fourth wider and in some specimens more brightly colored. The width of the underparts is likewise a fourth more than in _bangsi_. In _invicta_ the underparts are not so brightly colored as in some specimens of _richardsonii_. From _fallenda_, _richardsonii_ differs in that the upper parts often are lighter colored, upper lips white rather than colored like upper parts, and underparts as wide again. In comparison with _alascensis_, the black tip of the tail averages three-fifths rather than a half of length of tail-vertebrae. _Skull._--Male (based on 6 adults from 3 miles south of Big Island, Great Slave Lake): See measurements and plates 2-4; weight, 2.5 (2.1-2.9) grams; basilar length, 40.9 (39.6-43.7); length of tooth-rows less than length of tympanic bulla; breadth of rostrum measured across lacrimal processes less than a third of basilar length; interorbital breadth less than distance between glenoid fossa and posterior border of external auditory meatus; zygomatic breadth less than distance between last upper molar and jugular foramen. Female (based on 4 adults: from Willow River, 1; Fort Resolution, 1; Athabasca Delta, 2; and 2 subadults, one from 3 mi. S Big Island and one from 15 mi. above Smith Landing): See measurements and plates 9-11; weight, 1.1 (0.9-1.4) grams; basilar length, 33.1 (31.5-34.2); length of tooth-rows less than length of tympanic bulla; breadth of rostrum less than 30 per cent of basilar length; interorbital breadth less than distance between glenoid fossa and posterior border of external auditory meatus; zygomatic breadth less than distance between last upper molar and jugular foramen. The skull of the female averages 56 per cent lighter than that of the male. Comparison of the skull with that of _arctica_, _polaris_, _semplei_, _kadiacensis_, _haidarum_, _cicognanii_, _bangsi_, _invicta_, _fallenda_, and _alascensis_ is made in the accounts of those subspecies.