noun

• (anatomy) A small bone (or bony structure), especially one of the three of the middle ear.
• (zoology) Bone-like joint or plate, especially:
• one of numerous small calcareous structures forming the skeleton of certain echinoderms, as the starfishes;
• one of the hard articuli or joints of the stem or branches of a crinoid or encrinite;
• one of the several small hard chitinous parts or processes of the gastric skeleton of crustaceans, as in the stomach of a lobster or crawfish.