From _richardsonii_, _haidarum_ differs in that skull of the male is actually larger in its anterior part (breadth of rostrum, interorbital breadth and orbitonasal length) but all measurements of other parts average less. In relation to the basilar length, the tympanic bulla is shorter but all other measurements are more. In the skull of the female, which is 23 per cent heavier, the width of the tympanic bulla and anteroposterior extent of the inner lobe of M1 are the same; in all other measurements the female of _haidarum_ is larger, and in relation to the basilar length all measurements are more except the depth of the skull at the anterior margin of the basioccipital and the width of the tympanic bulla, which are less. By actual weight the skull of the male is 25 per cent lighter and the skull of the female 24 per cent heavier than in _richardsonii_. From _fallenda_ and _anguinae_, _haidarum_ differs in that measurements of the skulls of both sexes either average more, or are uniformly more, with two exceptions. These are the lesser length and breadth of the tympanic bulla, in comparison with males of _fallenda_, and the dimensions of M1 which are about the same in all three races concerned. The pre-and interorbital parts are larger in relation to the remainder of the skull. The postorbital breadth is actually a third more than in _fallenda_. In relation to the basilar length, the tympanic bulla is shorter and the braincase deeper than in males of _anguinae_. The skull of the male is 27 per cent heavier than that of _fallenda_ and 58 per cent heavier than that of _anguinae_. The skull of the female is 59 and 50 per cent heavier than those of _fallenda_ and _anguinae_, respectively. Comparison of the skull with those of _alascensis_, _celenda_ and _seclusa_ has been made in the accounts of those subspecies.