PROC. BIOL. SOC. WASH. 102(2), 1989, pp. 507-514 REDESCRIPTION OF THE GARTER SNAKE THAMNOPHIS EXSUL ROSSMAN, 1969 (SERPENTES: COLUBRIDAE) Douglas A. Rossman, Ernest A. Liner, Carlos H. Trevifio, and Alan H. Chaney Abstract. — Thamnophis exsul, previously known only from the holotype from Coahuila, is redescribed on the basis of nine recently collected specimens from Nuevo Leon. An ontogenetic change in dorsal color pattern is apparent in the Nuevo Leon sample. Notes on the habitat and associated herpetofauna are also presented. Thamnophis exsul was described by Rossman (1 969) from a single specimen col-lected 17.7 km E, 5.6 km S San Antonio de las Alazanas, Coahuila, Mexico, by M. D. Sabath on 6 Jul 1961. Subsequent attempts to find additional specimens at or near the type locality by field crews from Louisiana State University and Texas A&M Univer-sity, as well as by E. A. Liner, were unsuc-cessful. In the summer of 1979, Trevifio sent Rossman two puzzling garter snakes (Universidad Autonoma de Nuevo Leon, UANL 2250-5 1) that had been collected in southern Nuevo Leon the previous fall and tentatively identified as T chrysocephalus. Rossman soon concluded that, despite striking differences in color pattern from the holotype of T exsul (Fig. 1), the Nuevo Leon snakes were more closely related to exsul than to any other known species of Tham-nophis. Subsequent collecting in southern Nuevo Leon by E. A. Liner, A. H. Chaney, and the late R. M. Johnson has yielded sev-en additional specimens; these animals shed new light on the previously noted pattern differences, and it now seems almost certain that the Coahuila and Nuevo Leon snakes are conspecific. The purpose of this article is to redescribe what remains the rarest species of garter snake. Thamnophis exsul Rossman, 1969 Figs. 1-2 Specimens examined.— Coahuila: 17.7 km E, 5.6 km S San Antonio de las Alazanas (holotype, National Museum of Natural History, USNM 166423). Nuevo Leon: Zaragoza, Rancho La Encantada (UANL 2250-5 1); 1 9.6 km NE San Antonio de Pefia Nevada (Ernest A. Liner, EAL 4820, 4837; Louisiana State University Museum of Zo-ology 43836); 17.9 km NE San Antonio de Pefia Nevada (EAL 4971, 4 specimens). Definition. —The smallest species of Thamnophis (maximum recorded snout-vent length = 360 mm), T. exsul is char-acterized by having: a maximum of 1 7 dor-sal scale rows (in 90% of specimens), usually with no posterior reduction to 15 rows (80%); 7 supralabials on at least one side of the head (90%); 8 infralabials on at least one side (90%); 142-150 ventrals in 9$, 152-156 in 66; 52-56 subcaudals in 99, 63-65 in 66; tail length 18.9-20.0% of total length in 99, 21.8-22.2% in 66; 19-21 maxillary teeth, the last 2 or 3 slightly enlarged; the dorsum almost uniformly putty brown with 3 in-distinct light stripes, or having 3 or 4 alter-nating rows of spots or blotches with the light stripes partially or completely sup-pressed.