_Skull and teeth._--Male (based on type and 2 subadults): See measurements and plates 16-18. As described in _M. f. noveboracensis_ except that: Weight, 4.0 (3.7-4.3) grams; basilar length, 43.5 (43.3-43.6); zygomatic breadth not less than distance between anterior palatine foramen and anterior margin of tympanic bulla; postorbital breadth more than length of upper premolars; interorbital breadth more than distance between foramen opticum and anterior margin of tympanic bulla; least width of palate more or less than length of P4; tympanic bulla longer than rostrum. Female: Typical skull unknown. The skull from 12 miles east of Eagle Lake, Texas, lacks the convexity in the dorsal longitudinal axis and the skull agrees with those of larger individuals of _primulina_ except that the anteromedial faces of the tympanic bullae are less inflated, and the mastoid and zygomatic breadths are greater than in any female seen of _primulina_. Probably this greater breadth is the result of intergradation with _M. f. frenata_ to the westward. Compared with the skull of _M. f. olivacea_ that of _arthuri_ differs as follows: Averaging smaller in every part measured; basilar length 5 mm. less; by weight a fourth lighter; relative to basilar length, interorbital breadth greater and zygomatic and especially mastoid breadth less; dorsal outline of skull more convex in longitudinal axis; tympanic bullae narrower and less inflated especially on anteromedial faces. Compared with the skull of _noveboracensis_ that of _arthuri_ has the zygomatic breadth equal to or exceeding the distance from the anterior palatine foramen to the anterior margin of the tympanic bulla, whereas the zygomatic breadth is less than this distance in _noveboracensis_. Also, in _arthuri_, the rostrum is relatively shorter, the braincase is more inflated anteriorly, the zygomatic arches are more uniformly spreading, and the dorsal outline of the skull is distinctly convex, both transversely and longitudinally, whereas it is transversely more nearly flat in _noveboracensis_ and longitudinally is concave in the interorbital region. Compared with _M. f. primulina_, _arthuri_ has narrower bullae, which are much less inflated on their anteromedial faces, a less marked postorbital constriction, a braincase which is narrower across the mastoid region and broader anteriorly, and a skull, which, in longitudinal axis, has the dorsal outline markedly more convex.