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'.

Alternative forms for keel : keel base, keeled, keels, shell keel, tail keel.