Compared with the skull of _spadix_, that of the male, and the female, of _primulina_ averages smaller in every part measured. Expressed in percentages of the basilar length, the two depth measurements of the skulls are not significantly different, but, excluding the measurements of the bullae and teeth, the other cranial measurements are less. The main difference in relative proportions is in the tympanic bullae which average only a half millimeter shorter in males of _primulina_ and one and one-tenth millimeters shorter in females. The bullae are, therefore, relative to the basilar length, longer in _primulina_. The skull of _primulina_, then, differs from that of _spadix_ mainly in smaller size and relatively longer tympanic bullae, especially in males.
Compared with the skull of _M. f. longicauda_, that of both sexes of _primulina_ averages smaller in every part measured, except in males where the length of the tympanic bulla, and breadth and length of M1 are the same or slightly larger in _primulina_. Relative to the basilar length, the length of the tympanic bullae, and in females only, the depth measurements are greater in _primulina_ but all the others, in both sexes, are less. These ratios reflect the relative narrowness of the skull of _primulina_. Upon direct comparison the narrowness is especially noticeable in the interorbital region, mastoid region, tympanic bullae, and across the zygomata.
Compared with the skull of _M. f. neomexicana_ that of both sexes of _primulina_ averages smaller in every part measured. Excepting the measurements of the teeth, most of the other measurements are constantly larger. Relative to the basilar length, the length of tooth-rows and length of tympanic bulla are more, but excepting the depth measurements, the others are less. Still other differences are, in _primulina_, less well-developed sagittal crest, anterolateral corner of bulla rounded rather than "square," and in males a transversely convex rather than flat interorbital region.
Compared with _M. f. frenata_ and _M. f. texensis_, the skulls of males of _primulina_ differ in being smaller in every part measured but relative to the basilar length, have longer tooth-rows, a lesser zygomatic breadth and are less constricted interorbitally.