the smooth pyramidal or cone-shaped bone projection at the base of the skull on each side of the head just below and behind the ear in humans. The mastoid process is important to students of fossil humans because it occurs regularly and in the specific form described only in hominids (i.e., members of the genera Homo and Australopithecus). The development of the mastoid process is apparently related to the upright posture of hominids and the consequent evolutionary realignment of the head in relation to the neck.