_Range._--The species is known only from three localities on the Pacific drainage of southwestern Chihuahua; the geographic range probably includes parts of southern Sonora and northern Sinaloa. The discovery of _Geophis_ in southern Chihuahua increases to 21 the number of species of the genus known to occur in Mexico and extends the known range of the genus approximately 560 miles northwestward from the type locality of _G. latifrontalis_, a point 50 miles south (in Guanajuato?) of San Luis Potosí, or, a slightly lesser number of miles north-northwestward from an indefinite locality for _G. bicolor_ in western Jalisco (La Cumbre de los Arrastrados) (Boulenger, Catalogue of the snakes in the British Museum, Vol. 2, 1894, p. 298). _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 OF _Geophis aquilonaris_ NEW SPECIES. ====+======+========+======+========+==========+==========+==========+====== | | Sex |Dorsal|Ventrals|Subcaudals|Pale rings|Snout-vent|Length | | |scale | | +----+-----+ length | of | | |rows | | |Body|Tail | | tail ----+------+--------+------+--------+----------+----+-----+----------+------ UMMZ|111501|[Female]| 15 | 173 | 55 | 29 | 12 | 237 | 60 UMMZ|111502|[Female]| 15 | 181 | 58 | 34 | 14 | 355 | 88 UMMZ|117771|[Female]| 15 | 182 | 55 | 39 | 16 | 371 | 90 KU |44265 |[Female]| 15 | 183 | 63 | 38 | 17 | 327 | 93 KU |44266 |[Male] | 15 | 175 | 60 | 30 | 13 | 160 | 43 UMMZ|117770|[Male] | 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.