The Collaborative International Dictionary of English v.0.48:

Petrosal \Pe*tro"sal\, a. [See Petrous.] (Anat.)
   (a) Hard; stony; petrous; as, the petrosal bone; petrosal
       part of the temporal bone.
   (b) Of, pertaining to, or in the region of, the petrous, or
       petrosal, bone, or the corresponding part of the temporal
       bone.
       [1913 Webster]

   Petrosal bone (Anat.), a bone corresponding to the petrous
      portion of the temporal bone of man; or one forming more
      or less of the periotic capsule.
      [1913 Webster]