Variation.—Standard counts of scales are given for the paratypes as well as the holotype in table 1. The fifth and sixth supralabial scales are fused on both sides of one specimen (UMMZ 117771) as is the case on the left side of the holotype. Except for one specimen (UMMZ 117770) that has a small anterior temporal separating the posterior two-thirds of the fifth supralabial from the parietal on each side, the temporal formula in the type series is uniformly 0 + 1.
Table 1.—Counts of Scales, Measurements, and Other Data Pertaining to Holotype and Paratypes ofGeophis aquilonaris
New Species.
| Pale rings | |||||||||
| Sex | Dorsal scale rows | Ventrals | Sub- caudals | Body | Tail | Snout- vent length | Length of tail | ||
| UMMZ | 111501 | ♀ | 15 | 173 | 55 | 29 | 12 | 237 | 60 |
| UMMZ | 111502 | ♀ | 15 | 181 | 58 | 34 | 14 | 355 | 88 |
| UMMZ | 117771 | ♀ | 15 | 182 | 55 | 39 | 16 | 371 | 90 |
| KU | 44265 | ♀ | 15 | 183 | 63 | 38 | 17 | 327 | 93 |
| KU | 44266 | ♂ | 15 | 175 | 60 | 30 | 13 | 160 | 43 |
| UMMZ | 117770 | ♂ | 15 | 174 | 64 | 34 | 15 | 245 | 74 |
Considerably more variation occurs in color and in the arrangement of markings than in squamation. The ground color of the two specimens from Creel is black with little or no trace of brown, and the rings are white. Ground color in the remaining paratypes ranges from grayish black, with some brownishness on the belly, to dark brown, the colors in one specimen approximating the range from Mummy brown to Dresden brown, becoming paler posteriorly and ventrally. The head is slate gray to blackish brown in all the specimens. Those having a suggestion of brown on the head tend also to have more brown on the body.
The dark band on the neck is complete in four of the paratypes and incomplete (as in holotype) in one. Pale marks on the prefrontals are lacking in three of the paratypes and the pale mark between the parietals is lacking in two specimens (fused with white band on neck of one specimen). Pale postnarial crescents are evident in three paratypes.
As stated above, the rings on the holotype are mostly complete. Exceptions occur between the 13th and 15th white rings where two black rings are fused on the left side, rendering one black and one white ring (the 14th) incomplete. Also, where the ninth and tenth white rings fuse on the left side, they enclose a black ring and render it incomplete. The markings of the three smaller paratypes are substantially the same as those of the holotype—complete rings with a small number of variations in each specimen. In the two largest paratypes nearly all the white rings are incomplete ventrally, appearing to have been encroached upon by the darker ground color. In the larger speicmens there is a tendency also for the white rings to be one scale wide (rather than alternately one and two scales wide) and to lack a zigzag appearance; this appears to be due to the darkening of entire scales and to the darkening of the edges of other scales.
Relationships.—Geophis aquilonaris is distinct from all other Mexican representatives of the genus in having, on the body and tail, numerous, alternating pale and dark bands. Both sets of bands are in the form of complete rings or the dark bands are joined ventrally rendering the belly dark.
Of the seven other Mexican Geophis having 15 rows of scales, four species (cancellatus, dugesii, chalybeus, and semidoliatus) have alternating pale and dark transverse markings and therefore superficially resemble aquilonaris. Of the latter two species, the poorly known G. chalybeus (Veracruz) has a much lower (137 to 142) number of ventrals than aquilonaris, and G. semidoliatus (southeastern Mexico—Veracruz, Hidalgo, and Oaxaca) has a narrower head, fewer supralabials (four to five with only the third entering the orbit), and fewer ventrals (136 to 169) than aquilonaris. Geophis aquilonaris seems to be most closely allied to G. cancellatus (Chicharras, Chiapas) and G. dugesii (known from two localities in northern Michoacán); all three species resemble one another in the number and arrangement of the scales of the head, in general coloration, and in having relatively high numbers of ventral scales (171 in cancellatus, 150 to 164 in dugesii). G. cancellatus differs from the other two species in lacking internasal scales. Geophis aquilonaris differs from both species in having a higher number of ventral and subcaudal scales, a longer tail (tail contained in snout-vent length three to four times in aquilonaris, four and one half to six times in dugesii, 11 to 12 times in cancellatus), and in having more bands on the body (28 to 32 in cancel latus, two to seven in dugesii). The belly in dugesii and cancellatus is pale but in aquilonaris it is ringed or of a solid dark color.
As more specimens of Geophis become available from intermediate localities in Mexico, it will perhaps be demonstrated that many of the kinds now thought of as full species (including those discussed above) are subspecies of a few wide-ranging species.
Remarks.—The type locality of G. aquilonaris is the small village of Barranca at the bottom of the valley of the Río Urique, several miles south and west of the continental divide. The Urique Valley, known as the Barranca del Cobre in the region south of Creel, is a deep canyon, the walls of which slope abruptly from approximately 7300 to 3000 feet and are dissected by deep side-canyons. Coniferous forest on the upper rim of the canyon is replaced by scrub vegetation on the rocky walls and by an arid tropical flora on the bottom.