• (zoology) One of the lateral prominence just above the hinge of a bivalve shell.
  • (biology) A boss, or rounded elevation, or a corresponding depression, in a palate, disk, or membrane; as, the umbo in the integument of the larvæ of echinoderms or in the tympanic membrane of the ear.
  • (historical) The boss of a shield, at or near the middle, and usually projecting, sometimes in a sharp spike.
  • (mycology) A bump or protrusion on the top of the cap.

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