The Collaborative International Dictionary of English v.0.48:

petrohyoid \pet`ro*hy"oid\ (p[e^]t`r[-o]*h[imac]"oid), a. [Petro
   + hyoid.] (Anat.)
   Pertaining to the petrous, or periotic, portion of the skull
   and the hyoid arch; as, the petrohyoid muscles of the frog.
   [1913 Webster]