rib [paleont] . (a) A radial or transverse fold upon a shell; e.g. any radial ornament on a brachiopod, pelecypod, or rostroconch shell, or a raised radial ridge on the coiled conch of a nautiloid or ammonoid. (b) One of the arcuate, paired bones of the axial skeleton that connects dorsally to the vertebral column and ventrally to the sternum and helps support the body wall in Osteichthyes and tetrapods.