Between the pro-otic and opisthotic as seen in a longitudinal section of the skull is a large opening constricted in the middle. This is the =internal auditory meatus= (fig. 39, VIII.). Through it the auditory nerve leaves the cranial cavity and enters the ear. The ramus vestibularis leaves through the dorsal part of the hole, the ramus cochlearis through the ventral. The cavity of the auditory or periotic capsule communicates with the exterior by a fairly large hole, the =fenestra ovalis=, which lies between the opisthotic and pro-otic, and opens into a deep depression, the =tympanic cavity=, which is seen in a posterior view of the skull lying just external to the exoccipital. The cavity communicates with the exterior by a large opening, the =external auditory meatus= (fig. 38, 22). Several other openings are seen in the tympanic cavity; through one at the extreme posterior end the pneumogastric and spinal accessory nerves leave the skull, and through another, a little further forwards, the glossopharyngeal. The auditory ossicles consist of a long bony =columella=, whose inner end fits into the fenestra ovalis, while the outer end is attached to a small cartilaginous plate, the =extra-columella=, which is united to the tympanum. The =Optic capsules=. The skeletal structures developed in connection with the optic capsule do not become united to the skull. They consist of:-- (_a_) the =sclerotic=, a cartilaginous sheath investing the eye and bearing (_b_) a ring of ten small bony scales. There is no _lachrymal_ bone. The =Olfactory= or =Nasal capsules=. The basicranial axis in front of the basisphenoid remains cartilaginous, neither presphenoid nor mesethmoid bones are developed, and the orbits in a dry skull communicate by a wide space through which the second, third, fourth, and sixth cranial nerves pass out. Separate _nasal_ bones also do not occur, the large prefrontals extending over the area usually occupied by both nasals and lachrymals. The only bone developed in connection with the nasal capsules is the _vomer_ (fig. 39, 19), an unpaired bone lying ventral to the mesethmoid cartilage, and in contact laterally with the maxillae, premaxillae and palatines. 3. THE UPPER JAW AND SUSPENSORIAL APPARATUS. A number of pairs of bones are developed in connection with the upper jaw and suspensorial apparatus, one pair, the =quadrates=, being cartilage bones, while the rest are all membrane bones. The _squamosals_ (fig. 38, 2) are large bones which, lying external to the auditory bones, extend dorsalwards to meet the parietals and postfrontals, and form a large part of the false roof of the skull. They are united ventrally with the quadrates and quadratojugals.