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in vertebrates, a part of the skull that protects the inner ear. In cyclostomes, chondrichthians, sturgeons, and dipnoans the auditory capsule remains cartilaginous. In other fishes, it is formed from at least three of the following five bones: the anterior prootic, posterior opisthotic, dorsal epiotic, sphenotic, and pterotic bones. Modern amphibians usually have only one anterior prootic bone; the posterior half of the auditory capsule is formed by the lateral occipital bones. In reptiles and birds, in addition to the anterior prootic bone, posterior opisthotic and dorsal epiotic bones may develop; the last two bones usually fuse with the occipital bones. In mammals, two ossifications of the auditory portion of the skull are characteristic: the anterior prootic and posterior opisthotic bones, respectively called the petrous and mastoid bones. The posterior part of the petrous bone in man forms the mastoid process.