1. In corals, an external projection of a septum beyond a corallite wall, forming a rib.

2. A rib-like thickening of a shell, extending from the umbo to the margins in brachiopod (Brachiopoda) and lamellibranch (Bivalvia) shells. In gastropods (Gastropoda) the thickening of the shell usually runs either axially or spirally, and thus there may be axial or spiral costae.