The =quadrate= (fig. 60, 12), which unites the two arcades behind, is a stout irregular four-cornered bone forming the =suspensorium=. It articulates by its dorso-posterior corner with the squamosal, and by its antero-internal corner with the pterygoid. The middle of its ventral surface forms a hemispherical knob with which the mandible articulates, while its dorso-anterior border is drawn out into a long point which extends towards the interorbital septum. (3) THE MANDIBLE. The =mandible= or lower jaw consists of two =rami= which are flattened and fused together in the middle line in front, while behind they diverge from one another and articulate with the quadrates. Each ramus is composed of five bones fused together, one being a cartilage bone, and the other four membrane bones. The =articular= is the only cartilage bone of the mandible, it bears the double condyle (figs. 59 and 60, 13) or concave articular surface for the quadrate, and is drawn out behind into a large hooked =posterior articular process=. The articular is also drawn out into a prominent process on each side of the articular surface for the quadrate, and is marked by a deep pit opening posteriorly. The articular is continuous in front with =Meckel's cartilage= which forms the original cartilaginous bar of the lower jaw, and is ensheathed by the membrane bones. Of these the _supra-angular_ forms the upper part of the mandible in front of the articular, its dorsal surface is drawn out into a small =coronoid process=, its outer surface also bearing a prominent process. The _angular_ is a small bone which underlies the articular and supra-angular on the inner side of the jaw. The _dentary_ (fig. 59, 15) forms the anterior half of each ramus, and is the largest bone of the mandible; it is fused with its fellow at the symphysis in front, and extends back below the supra-angular. The _splenial_ is a small bone lying along the middle half of the inner side of each ramus of the mandible. (4) THE HYOID. With the hyoid apparatus is included the =columella=. This forms a minute rod of bone, one end of which is expanded and fits into the fenestra ovalis, while the other end, terminated by a triradiate piece of cartilage, is attached to the tympanic membrane. The structure is as a whole homologous with the auditory ossicles of mammals and the hyomandibular of fish. The =hyoid= consists of a median unpaired portion, formed of two pieces of bone, the =basi-hyal= (fig. 59, C, 16) in front, and the =uro-hyal= (fig. 59, C, 17) behind, the two being placed end to end and terminated anteriorly by an unpaired cartilaginous plate, the =os entoglossum.= At the posterior end there come off a pair of long =posterior cornua=, each of which consists of two pieces, a longer =basibranchial= (fig. 59, C, 18), and a shorter =cerato-branchial=. For the homology of these parts see p. 336. THE RIBS AND STERNUM.