[Foraminifera] a peripheral crest along the outer margin of the chambers, usually thickened by additional calcite deposition (Fo5c).

[Acantharia] Sail-like longitudinal plate extending laterally from spicule or bar (see ribs, wing).

[Gastropoda general] calcareous or cartilaginous flat ridge in heteropods on the outside of the shell along the body whorl, the line where it reaches the main shell is the keel base; sometimes 'keel' creates a sharp ridge over the body in Carinariidae and Pterotracheaidae and especially over the tail the 'tail keel'.

[Cephalopoda] A flattened, muscular extension along the aboral surface of some arms or the tentacular club to render them more hydrodynamic (Ce 1C,Ce 1E) (see also swimming membrane).