The Collaborative International Dictionary of English v.0.48:

Malleus \Mal"le*us\, n.; pl. Mallei. [L., hammer. See Mall a
   beetle.]
   1. (Anat.) The outermost of the three small auditory bones,
      ossicles; the hammer. It is attached to the tympanic
      membrane by a long process, the handle or manubrium. See
      Illust. of Far.
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   2. (Zool.) One of the hard lateral pieces of the mastax of
      Rotifera. See Mastax.
      [1913 Webster]

   3. (Zool.) A genus of bivalve shells; the hammer shell.
      [1913 Webster]