In order to study certain variations, the following "method of pairs" was used. Skulls of two series to be compared were matched in pairs so that they corresponded in size and ontogenetic stage of development. Then the two skulls of each pair were examined for differences in each of the following features: size of circle inscribed by the upper incisor teeth, width of nasal bones relative to their length, curvature of the zygomatic arch, elongation of the braincase relative to its width when viewed from the dorsal aspect, degree of indentation in the anterior edge of the zygomatic arch near the rostrum, degree of depression of the nasal bones when viewed from the side, width in the vertical plane of the zygomatic arch at the suture between the maxillary and jugal bones, length relative to width of the prominent fenestra in the posterodorsal part of the squamosal bone, size of the meatus of the auditory canal, distance between the internal margin of an occipital condyle at its posteriormost point and the tip of the paraoccipital process of the same side of the skull, size of the foramen magnum, vertical height of the supraoccipital bone from the dorsalmost point on the margin of the foramen magnum to the midpoint of the lambdoidal crest, constriction posteriorly or narrowness of the incisive foramen relative to its length, distance across the premaxillary bone from the anteriormost point of the incisive foramen to the posteriormost point of the margin of the alveolus of the upper incisor, area of the maxillary septum (Howell 1926:112, or "zygomatic plate" of Ellerman 1941:1), acuminateness of the anterior border of the palatine opening (internal nares), size of auditory bullae, size of foramen ovale, acuteness of the angle between the basioccipital and basisphenoidal bones at the suture between them (degree to which the area of the suture is raised between the bullae when viewed from the ventral aspect), width of first upper molar tooth, least distance between alveoli of first upper molars. Any differential feature present in more than 75 per cent of the pairs of animals is reported in the discussion of the subspecies concerned. The significance of each difference reported was calculated by the Chi-square test and the confidence limit is given in each case. The probability used in the Chi-square formula is one-half of the percentage of all pairs compared in which the skulls were different in regard to the character being considered. For example, in 68 per cent of the total number of pairs of skulls compared in this study a difference in the size of the auditory bullae was noted. Therefore the probability that a specified skull of a pair will have larger bullae than the other skull was taken as 34 per cent. A different probability for each feature compared was derived in like manner.