The characteristics of the genus in combination with the reduction of dark coloration posterior to the head distinguish this snake from all other snakes in México and Central America.
_Distribution._--The Yucatán Peninsula: eastern Campeche, all of Yucatán, probably in Quintana Roo, and the northern third of British Honduras. A record for northeastern Honduras is questioned (fig. 2).
_Specimens examined._--Total of 48, as follows: BRITISH HONDURAS: _Belize District_: 13.0 mi. W, 1.5 mi. S Belize, ERA-WTN BH-1562.
GUATEMALA: _El Petén, no specific locality_, USNM 4941.
MÉXICO: _Campeche_: Champotón, UMMZ 73063-66; Encarnación, CNHM 106462. _Yucatán: no specific locality_, BMNH 80.7.13.30; Chichén Itzá, AMNH 38826, 38833, CNHM 20610-11, 26986-87, 36299-300, 36303-04, 36307, 36316, MCZ 7422, 28748, UMMZ 68236, 73060-62, 80806, USNM 46395; Kantunil, CNHM 36301, 36305-06, 36308-09, 36312-13; _Libré Union_, CNHM 36298, 36302, 36310-11, 36314; Mayapán, CNHM 40720; Mérida, CNHM 19411, 19413, NMW 16828; Progreso, CNHM 40721; Tekom, CNHM 49374; Yokdzonot, CNHM 36315.
_Coniophanes imperialis imperialis_, Wettstein, Sitz. Akad. Wiss. Wien. mathem-naturw. Kl., 143:37-38, 1934.
_Conophis nevermanni_ Dunn, Copeia, no. 4:214, December 31, 1937; Smith, Proc. U. S. Natl. Mus., 92:395, November 5, 1942; Savage, Trans. Kansas Acad. Sci., 50:484, December 31, 1949; Taylor, Univ. Kansas Sci. Bull., 34(pt. 1): 145-146, October 1, 1951.
_Type._--Academy of Natural Sciences of Philadelphia, no. 22423, obtained by Emmet R. Dunn from Prof. Manuel Valerio. Type locality: Río Poas de Aserri (a few miles south of San José), Costa Rica.
_Diagnosis._--Head and body dark brown or black above with two or four white stripes along body; usually two white lines on head immediately above eye passing from canthus rosetralis posteriorly to connect with white stripe on 6th row of dorsal scales; eight supralabials with black margins above.
_Variation._--Six specimens have 173 to 183 (176.5 ± 4.00) ventrals. Five of these snakes having complete tails have 71 to 89 (80.6 ± 7.15) subcaudals; the number of ventrals plus subcaudals varies from 250 to 263 (257.0). In the six specimens the reduction from 19 to 17 dorsal scales takes place between ventrals 84 and 97 (93.2 ± 4.71). Sexual dimorphism is evident in the number of subcaudals; two females have 71 and 76 (73.5), and three males have 82 to 89 (85.3) subcaudals. The longest specimen (ANSP 22424) is a female from San José, Costa Rica, having a body length of 660 mm., a tail length of 168 mm. and a total length of 828 mm.
