Aside from one specimen from Temazcal and nine from Morelia (paratypes of T. vicinus), only two other specimens completely lacking the middorsal stripe have been seen; one is a male (UMMZ 102510) having 161 ventrals and an incomplete tail from Pino Gordo, and the other is a male (CNHM 39060) from Tancítaro having 158 ventrals and an incomplete tail. A female from Tancítaro (CNHM 39061) having 153 ventrals and 77 caudals has no lateral stripes and only a narrow middorsal stripe on the anterior part of the body. Throughout the region where T. vicinus-like snakes have been found, typical T. dorsalis cyclides occurs in much greater numbers. I concur with Bogert and Oliver in placing T. vicinus as a synonym of T. dorsalis cyclides.
Fig. 10. Dorsal color pattern of Thamnophis dorsalis cyclides (A) and Thamnophis dorsalis postremus (B).
Milstead (1953) had available few specimens of Thamnophis dorsalis from the Tepalcatepec Valley. The large series now in existence shows that the population in the Tepalcatepec Valley differs distinctly from that inhabiting the Mexican Plateau, Cordillera Volcánica, and Sierra de Coalcomán. Therefore the name T. dorsalis postremus Smith (1942) is resurrected for the population in the Tepalcatepec Valley. T. dorsalis cyclides and T. dorsalis postremus differ in color pattern (Fig. 10) and in scutellation (Table 6). Specimens from the Mexican Plateau and mountain ranges have a distinct light stripe on the second and third scale rows, a dark brown dorsum having squarish black spots, and a row of dark spots on the first row of dorsal scales. Specimens from the Tepalcatepec Valley have a grayish brown dorsum having smaller and less distinct dark spots and no light stripe on the second and third scale rows; the first, second, and third rows of scales are colored like the venter. In some specimens there are small dark flecks on the first row of dorsal scales.
Table 6.—Variation in Scutellation in Thamnophis dorsalis.
| Character | Mexican Plateau | Sierra de Coalcomán | Tepalcatepec Valley | |
| Ventrals | ♀ N | 31 | 2 | 32 |
| Mean | 164.0 | 156.5 | 144.6 | |
| Range | 153-171 | 154-159 | 138-151 | |
| ♂ N | 19 | 2 | 32 | |
| Mean | 153.5 | 154.7 | 138.3 | |
| Range | 149-159 | 149-159 | 131-141 | |
| Caudals | ♀ N | 28 | 2 | 29 |
| Mean | 83.8 | 81.0 | 73.4 | |
| Range | 80-100 | 79-83 | 70-79 | |
| ♂ N | 14 | 2 | 28 | |
| Mean | 78.0 | 72.0 | 68.5 | |
| Range | 71-87 | 72 | 63-73 | |
One specimen from Uruapan (1550 meters) and one from Coalcomán (950 meters) are intermediate in color pattern between T. dorsalis cyclides and T. dorsalis postremus. Both have indistinct lateral stripes and only small dark spots below the stripes. In scutellation these specimens are like T. dorsalis cyclides.
In Michoacán Thamnophis dorsalis cyclides has been collected in a variety of habitats on the Mexican Plateau: pine-oak forest, fir forest, marshes, and cleared land from 1550 to 2800 meters. In the Sierra de Coalcomán one was taken in broad-leafed forest at 950 meters, three in pine-oak forest at 2100 meters, and one in pine forest at 2300 meters.
Thamnophis dorsalis postremus Smith
Thamnophis eques postremus Smith, Zoologica, 27:109, October 23, 1942.—El Sabino, Michoacán, México.