Comparison of the skull with that of M. f. tropicalis is made in the account of that subspecies. Compared with the skull of M. f. macrophonius, that of the female of perotae is more flattened, has the longitudinal dorsal outline distinctly concave rather than flat just behind the postorbital processes, and much wider tympanic bullae. Accordingly, the basioccipital is slightly narrower in perotae. The more marked postorbital constriction of the type specimen of perotae possibly is due to its relatively greater age. As compared with the skull of M. f. leucoparia, that of the female of perotae has less inflated tympanic bullae, the height of each being half as great as distance from its anterior margin to foramen ovale, whereas, in leucoparia (as represented by no. 26153) the two distances are equal. As compared with that of M. f. frenata, the skull of the female of perotae differs mainly in the lesser inflation of the tympanic bullae and their relative position. The height of each bulla is in perotae only half as much as, but in frenata more than, the distance from its anterior margin to foramen ovale. The anterior margin of the bulla is much less projected from the floor of the braincase in perotae. The squamosal anterior to each bulla is convex ventrally in perotae but flat or concave ventrally in frenata.
Remarks.—The type specimen and a juvenal female from the town of Perote were taken in the spring of 1893 by E. W. Nelson. Of these two, the type specimen was mentioned and figured by Merriam (1898:30, fig. 16 [= fig. 15], pl. 3, fig. 2) as Putorius frenatus. The referred nontypical specimen from Cerro San Felipe, Oaxaca, was referred by Merriam (op. cit.:29) to Putorius frenatus goldmani with the comment that it was intermediate ". . . both in coloration and cranial characters, between typical frenatus and goldmani;. . . ." No other published references to this subspecies, or specimens of it, have been seen. In 1941 and 1942, W. B. Davis and associates took four specimens along the boundary between the states of Puebla and México.
Although the specimen from Cerro San Felipe, Oaxaca, is referred to Mustela frenata perotae, to the description of which it answers best, that specimen, on account of its structural characters and geographic position relative to adjacent races, is in reality an intergrade between several of the adjacent races. Some of its intermediate characters are pointed out in the discussion of M. f. goldmani. In the specimens from 45 and 55 kilometers ESE of México (city) the black color of the top of the head does not extend so far behind the ears as in the holotype of M. f. perotae and in this feature the two specimens show intergradation between the two subspecies, perotae and frenata.
The type specimen taken on May 26, is acquiring new hair on the belly and lower sides which appears to be the result of a normal molt.
As would be expected from its geographic position, M. f. perotae resembles M. f. frenata of northern México and the high mountain forms of southern México more than it does the lowland tropical forms. This is true as regards size of entire animal, proportions of its parts, and size, general angularity and major proportions of its skull. The marked postorbital constriction, convex supralacrymal face of rostrum, width of tympanic bullae and angularity of the braincase place it nearest M. f. frenata as does also the color and color pattern. The ventrally convex squamosal anterior to each tympanic bulla and the slight degree of projection from the cranium of the anterior margin of each tympanic bulla are intermediate in degree between the condition in M. f. macrophonius and that in M. f. frenata. Thus M. f. perotae combines several characters of M. f. frenata on the one hand with some of M. f. macrophonius on the other and in some features, for instance in the size, shape and degree of inflation of the tympanic bullae, presents intermediate stages of development.
On the eastern plain below the high mountain, Cofre de Perote, there ranges the similarly colored, smaller, tropical weasel, Mustela frenata tropicalis. Between M. f. perotae and M. f. tropicalis there is marked differentiation in the skulls with much less differentiation in coloration. The differences in typical skulls of the two subspecies are so pronounced that one would, at first glance, hardly believe it possible for direct intergradation to occur between them on the sides of this mountain. Merriam (1896:30) thought that it did not. The two skulls figured by him (op. cit.:31) are a topotype of M. f. tropicalis from Jico and the one which now is the type specimen of M. f. perotae. They show the great difference in size and proportions and are females of comparable ages, not of different ages as I suspected before examining the skulls. However, despite this marked difference in the skulls, there is some, although not conclusive, evidence of intergradation furnished by a young female from Xuchil, Veracruz. This specimen is described in connection with M. f. tropicalis (see page [366]).
None of the seven skulls shows marked deformity of the interorbital region, but two of the three adults appear to have had these parts infested with nematodes.
Specimens examined.—Total number, 7, all from México, listed by localities from north to south. Specimens from Veracruz and Oaxaca in the United States National Museum; remainder in Texas Cooperative Research Collection.
México: Monte Río Frío, 45 Km. ESE México City, 1; 55 Km. ESE México City, 1; N slope Mt. Popocatépetl, 13,555 ft., 1.
Puebla. Río Otlati, 8700 ft., 1.
Veracruz. Cofre de Perote, 12,500 ft., 1; Perote, 1.
Oaxaca. Cerro San Felipe, 10,000 ft., 1.