Systematics of Neotropical gastropod-eating snakes: the dimidiata group of the genus Sibon, with comments on the nebulata group

AbstractSibon dimidiata and S. sanniola comprise the dimidiata group of Sibon, and S. nebulata and S. dunni the nebulata group. The species within each group are more closely related to each other than to any other species of Sibon. S. annulata, S. argus and S. longifrenis are placed in the synonymy of S. dimidiata, with no basis for recognition of the nominal subspecies S. dimidiata grandoculis. S. sanniola has two subspecies, the nominate form and S. sanniola neilli. The dimidiata group is distinguished by the shared characteristics of: (1) non-cylindrical body (laterally compressed or bread-loaf shape in cross section); (2) head distinctly wider than neck; (3) protruding eyes of large diameter; (4) vertebral and paravertebral scales not or only scarcely wider than other dorsals; (5) 11-20 maxillary teeth; (6) 20-27 dentary teeth; and (7) 1 or 2 postmental scales. The nebulata group is distinguished by the shared characteristics of: (1) laterally compressed body; (2) head distinctly wider than neck; (3) vertebral scales 1 1/4 -1 1/2 times wider than other dorsals; (4) about 12-17 maxillary teeth; (5) about 18-24 dentary teeth; and (6) absence of a postmental scale.