Fig. 4, 5A, Table 1

Schmidtophis rubriventris Taylor, 1949, Univ. Kansas Sci. Bull. 33: 169–215 [193–194]. Holotype: LSUMZ 577. Type locality: “near Xilitla (Xilitla Region), San Luis Potosí, Mexico.”

Diagnosis. Chersodromus rubiventris can be distinguished from all Mexican species of snakes by having combination of prefrontals fused into single scale; postocular discrete from supraocular; anterior temporal absent; dorsal scales keeled in 15 rows at midbody, unreduced posteriorly; supralabials usually 6, third and fourth entering orbit; infralabials 7–8, usually 1–5 contacting anterior chinshields (5 narrowly); mental not contacting anterior chinshields; venter bright red.

This species is easily differentiated from all species of Chersodromus by having 15 scales around body, mental scale in contact with anterior chinshields and anterior temporal absent (Table 1).

Description. Small, slender snake; head moderately wider than the neck. Adults reach 300–349 mm in total length, largest known male 319 mm, and the largest female 349 mm. Tail comprises 21–23% of total length in males, 17–21% in females.

Prefrontals fused into a single, large scale, supraoculars are not fused with the postoculars, frontal relatively broad, and the nasal divided. Small internasals 1/1, loreals 1/1 that extend from the postnasal to anterior edge of eye (no preoculars), discrete postoculars 1/1; temporals 0+1, supralabials 5–6 with 3–4 entering orbit, infralabials 6–7 with 1–5 contacting anterior chinshields (fifth pair narrowly and first pair in contact along midline behind mental). Mental broader than long, anterior chinshields large, pair of posterior chingshields about one-fourth to one-third size of anterior pair. Keeled dorsal scale rows 15-15-15, preventrals 2–3, ventrals 123–127 in males and 126–130 ventrals in females. Cloacal scute undivided. Divided subcaudals 41–45 in males and 37–41 in females. Dorsal scale rows at midlength of tail 6. Pupil is subcircular to vertically oval.

A conspicuous cream, yellow, or peach collar is present on the back of head. The anterior border is irregular, usually extending on top of the head across the anterior third of the parietals and then extending forward on side of head to about the postocular. The posterior border is located slightly anterior to the posterior edge of parietals; collar usually includes about half of the parietals and portions of the posterior temporal and posterior two supralabials. Collar sometimes reduced or broken with a central blotch and bars on each side of the head. Dorsum is blackish, narrowly extending to the lateral portion of ventrals and subcaudals. A black head cap extends from the rostral to about anterior the third of parietals, covering the internasals, a single fused prefrontal, supraoculars and fused postoculars, nasals, and loreal, and extending to the lower edge of supralabials 1–5. The mental and infralabials 1–5 (sometime more) have a suffusion of brown pigment. Except for dark lateral edging, the belly and subcaudals are immaculate bright red.

Maxillary dentition (Fig. 4D). Right maxilla extending anteriorly to level of suture between second and third supralabials, dorsoventrally compressed; in lateral view, heaviest at about midlevel of bone, tapering at anterior and posterior ends; anterior one-fourth of maxillary edentate, followed by 7–9 long, slender, curved teeth, decreasing in size posteriorly.

Hemipenis morphology (Fig. 4 E–G). Hemipenis cylindrical, unbifurcated, semicapitate on sulcate side, uniformly ornamented by spines, sulcus spermaticus bifurcate and centrifugal, sulcus spermaticus bifurcation point below mid-point of hemipenis body; intra-sulcar region protruding from hemipenis body and covered with spines decreasing in size distally. In asulcate view, hemipenis uniformly ornamented with spines decreasing in size distally. Medially, inverted “V” shaped depression coinciding with bifurcation level of sulcus spermaticus (sulcate side).

Distribution and habitat. This species is found in the Sierra Madre Oriental in the States of San Luis Potosí, Querétaro and Hidalgo (Fig. 1). Chersodromus rubriventris inhabits cloud forest between 700–1650 m asl, a habitat that has been fragmented in recent years (Ramírez-Bautista et al. 2013, Fig. 5B).