“The _tympanic cavity_, leading to the exterior by the _external auditory meatus_ (Fig. 21, 16), is well seen in a side view of the skull; it is bounded on its inner side by the periotic bones, posteriorly in part by the exoccipital, and elsewhere mainly by the quadrate. A large number of canals and passages open into it. On its inner side opening ventro-anteriorly is the _fenestra ovalis_, opening ventro-posteriorly the _internal auditory meatus_ (Fig. 23, VIII), while dorsally there is a wide opening which forms a communication through the roof of the brain case with the tympanic cavity of the other side. On its posterior wall is the prominent foramen through which the facial nerve passes on its way to its final exit from the skull through the exoccipital; this foramen is bounded by the quadrate, squamosal, and exoccipital. The opening of the _fenestra ovalis_ is in the fresh skull occupied by the expanded end of the auditory ossicle, the _columella_, whose outer end articulates by a concave facet with a trifid _extracolumellar_ cartilage which reaches the tympanic membrane. The lower process of this extracolumella passes into a cartilaginous rod which lies in a canal in the quadrate and is during life continuous with Meckel’s cartilage within the articular bone of the mandible. “The columella and extracolumella are together homologous with the chain of mammalian auditory ossicles.” _The Lower Jaw_ (Figs. 21, 23, and 24). The mandible consists of two similar rami, rather closely united at the anterior-median symphysis with each other. Each ramus consists of six bones. The _dentary_ (Figs. 23 and 24, 18; Fig. 21, 20) is a long bone that unites at the symphysis with its fellow to form the point of the jaw. It bears, along its dorsal edge, about twenty teeth; all but the posterior four or five of these teeth are in individual sockets; this may vary somewhat with age. The outer surface of the dentary, especially towards the symphysis, is covered with numerous, small, deep pits, while along its inner side, parallel to the row of teeth, is a row of somewhat larger pits like those noted in the maxilla and premaxilla. Articulating with the mesial side of the dentary along the greater part of its length is a flat bone, the _splenial_ (Figs. 23 and 24, 19); between these two bones is a long cavity that makes the ramus hollow almost to the symphysis. A large foramen, not shown in any of the figures, leads through the splenial into this cavity. Articulating with the caudal end of the splenial and forming the anterior border, as seen from the mesial side, of the large _external mandibular foramen_ (Fig. 23, 27) is a small bone, the _coronoid_ (Figs. 23 and 24, 23, Fig. 21, 21); it articulates with the splenial anteriorly, with the supra-angular dorso-caudally, and with the angular ventrally.