The cartilage of the lower jaw is in all animals with ossified skeletons, except the Mammalia, partly replaced by cartilage bone forming the =articular=, partly overlain by a series of membrane bones the _dentary_, _splenial_, _angular_, _supra-angular_ and _coronoid_. In many sharks large paired accessory cartilages occur at the sides of the jaws; and in a few reptiles and some Amphibia, such as the Frog, the ossified representative of the anterior of these structures occurs forming the =mento-meckelian= bone. In mammals the lower jaw includes but a single bone. The quadrate in all animals with ossified skeletons, except the Mammalia, forms the suspensorium of the mandible or the skeletal link between the jaw and the cranium; in the Mammalia, however, the mandible articulates with the squamosal, while the quadrate is greatly reduced, and is now generally considered to be represented by the tympanic ring of the ear. The second visceral or hyoid arch in fishes consists of two pieces of cartilage, a proximal[1] piece the =hyomandibular=, and a distal[10] piece the =cerato-hyal=. The cerato-hyals of the two sides are commonly united by a median ventral plate, the =basi-hyal=. The hyoid arch bears gills on its posterior border, but its most important function in most fishes is to act as the suspensorium. In higher vertebrates the representative of the hyomandibular is much reduced in size, and comes into relation with the ear forming the =auditory ossicles=; the cerato-hyal looses its attachment to the hyomandibular and becomes directly attached to the cranium, forming a large part of the hyoid apparatus of most higher vertebrates. Behind the hyoid arch come the branchial arches. They are best developed in fishes, in which they are commonly five in number and bear gills. Their ventral ends are united in pairs by median pieces, the =copulae=. In higher vertebrates they become greatly reduced, and all except the first and second completely disappear. In the highest vertebrates, the mammals, the second has disappeared, but in birds and many reptiles it is comparatively well developed. 3. THE RIBS AND STERNUM.