Compared with the skull of _M. f. noveboracensis_ from Massachusetts, that of the male of _primulina_, in dorsal view, is seen to be shorter anteriorly to the postorbital processes and to have a more marked postorbital constriction. In lateral view the dorsal outline of the skull of _primulina_ is less concave in the postorbital region. In ventral view the skull of _primulina_ is seen to be wider across the mastoid processes and zygomatic arches but the most pronounced difference is in the tympanic bullae. In _noveboracensis_ each bulla is scooped out on the anterior part of the medial face and appears to be narrower anteriorly than posteriorly whereas in _primulina_ the anterior part of the medial face is not scooped out but is moderately inflated and the bulla appears to be of uniform breadth anteriorly and posteriorly. By actual measurement the breadth of the bulla averages 59 per cent of its length in _primulina_ but only 50 per cent in _noveboracensis_. Other respects in which the skull of the male of _primulina_ differs from that of _noveboracensis_ are as follows: Linear measurements of teeth more; relative to the basilar length, the length of the tooth-rows averages more, whereas the interorbital breadth and orbitonasal length are less.
When skulls of females are compared, each of the differences mentioned above is found to apply, except that the degree of difference is in some parts greater, for example, in the tympanic bullae. In _primulina_, the bulla is in general like that of the male _noveboracensis_, whereas in the female _noveboracensis_ it is less inflated, especially anteromedially, shorter, relatively narrower, and in ventral view projects little or none below the squamosal floor of the braincase. The breadth of the bulla averages 51 per cent of its length in _primulina_ but only 47 per cent in _noveboracensis_. The bullae project below the basioccipital on the average, for a distance of 2.9 millimeters in female _primulina_ and only 2.3 millimeters in female _noveboracensis_. In _primulina_ the temporal ridges are well developed and fuse to form a low sagittal crest, but in _noveboracensis_ the ridges are absent. Also, in _primulina_ the mastoid processes project farther laterally beyond the braincase. The skull of female _noveboracensis_ is much lighter than that of _primulina_. Average weights of the two are 1.7 and 2.2 grams. The skulls of females of _primulina_ and _noveboracensis_ differ more than do the skulls of males.