Sibon argus Cope, 1875, long known only from the holotype, is redescribed based on material from Costa Rica and Panama. It differs from the only other member of the genus having an ocellate dorsal pattern (S. longifrenis) in its attenuate habitus, enlarged blunt head, protuberant eyes, and high segmental counts (ventrals 181-201, subcaudals 112-121, total segmental counts 294-312). Sibon longifrenis of Atlantic slope Costa Rica and western Panama (ventrals 151-173, subcaudals 82-103, total segmental counts 231-275) is also redescribed. These species differ from all other Sibon in having an ocellate pattern and an enlarged penultimate supralabial bordering the orbit. The allied species, S. annulatus (Costa Rica and Panama) and S. dimidiatus (Mexico to southwestern Costa Rica), are shown to be distinct from S. argus and S. longifenis in scalation and coloration. Although allopatric, S. annulatus and S. dimidiatus differ from one another most strikingly in adult coloration, postmental character states, and ventral counts (x = 175.8 in annulatus and 193.6 in dimidiatus), and are regarded as valid species. Sibon annulatus occurs sympatrically with S. argus in Panama and with S. longifrenis in Costa Rica. Although S. argus and S. longifrenis occur in the same general area on the Atlantic slope of Costa Rica, they have not yet been taken at the same locality.
