Source : Webster's Revised Unabridged Dictionary (1913)

Umbo \Um"bo\, n.; pl. L. {Umbones}, E. {Umbos}. [L.]
   1. The boss of a shield, at or near the middle, and usually
      projecting, sometimes in a sharp spike.

   2. A boss, or rounded elevation, or a corresponding
      depression, in a palate, disk, or membrane; as, the umbo
      in the integument of the larv[ae] of echinoderms or in the
      tympanic membrane of the ear.

   3. (Zo["o]l.) One of the lateral prominence just above the
      hinge of a bivalve shell.

Source : WordNet®

umbo
     n : a slight rounded elevation where the malleus attaches to the
         eardrum
     [also: {umbones} (pl)]

umbones
     See {umbo}