The Collaborative International Dictionary of English v.0.48:

Petrous \Pe"trous\, a. [L. petrosus, fr. petra a stone.]
   1. Like stone; hard; stony; rocky; as, the petrous part of
      the temporal bone. --Hooper.
      [1913 Webster]

   2. (Anat.) Same as Petrosal.
      [1913 Webster]

WordNet (r) 3.0 (2006):

petrous
    adj 1: (of bone especially the temporal bone) resembling stone
           in hardness [syn: petrous, stonelike]