The Collaborative International Dictionary of English v.0.48:

Mastoidal \Mas*toid"al\, a.
   Same as Mastoid.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mastoidal
    adj 1: relating to or resembling a nipple [syn: mastoid,
           mastoidal]
    n 1: process of the temporal bone behind the ear at the base of
         the skull [syn: mastoid, mastoid process, mastoid
         bone, mastoidal]