The periotic has its inner surface marked by important depressions, while both inner and outer surfaces are pierced by foramina. At about the middle of its inner surface are seen two deep pits, one lying immediately above the other. Of these the more ventral is a foramen, the =internal auditory meatus= (fig. 72, 20), through which the VIIth (facial) and VIIIth (auditory) nerves leave the cranial cavity, the facial nerve passing through the bone and afterwards leaving the skull by the =stylomastoid foramen= (fig. 75, VII), while the auditory passes to the inner ear. The more dorsal of the two pits is not a foramen but the =floccular fossa= (fig. 72, 23) which lodges the floccular lobe of the cerebellum. In some skulls another wide and shallow but fairly prominent depression is seen dorsal to and slightly behind the floccular fossa, this also lodges part of the cerebellum. Behind the internal auditory meatus, between the periotic and exoccipital is seen the internal opening of the =foramen lacerum posterius= (fig. 72, 22). The shape of this opening varies. The ventro-anterior border of the periotic is marked by a deep notch, the sides of which sometimes unite, converting it into a foramen. On the outer side of the periotic, and clearly seen only after the removal of the tympanic, are two holes, the =fenestra ovalis= and the =fenestra rotunda=. The _tympanic_ (figs. 72, 15 and 75, 4) is a greatly expanded boat-shaped bone, which forms the auditory bulla and lies immediately ventral to the periotic; it is separated from the periotic by the =tympanic cavity= into which the fenestra rotunda and the fenestra ovalis open. There are several other openings into the tympanic cavity. (_a_) On the external surface is a large oval opening, the =external auditory meatus= bounded by a thickened rim. (_b_) Into the outer and anterior part of the cavity the outer end of the =Eustachian tube= opens; while the inner end passes through a foramen (fig. 75, 22) just external to the foramen lacerum medium, on its way to open into the pharynx. (_c_) The internal carotid artery also enters the tympanic cavity by a canal which commences in the foramen lacerum posterius, and passes forwards to open on the inner side of the bulla. The artery then passes forwards, and barely appearing on the ventral surface of the cranium, enters the brain cavity through the foramen lacerum medium (fig. 75, 9). Immediately behind the tympanic, between it and the mastoid process of the periotic and the paroccipital process of the exoccipital is the =stylomastoid foramen= (fig. 75, VIII). Within the tympanic cavity are four small bones, the =auditory ossicles= (cp. fig. 74), called respectively the =malleus=, =incus=, =lenticular= and =stapes=; these together form a chain extending from the fenestra ovalis to the tympanic membrane.