The cartilaginous cranium formed of the basal plate, together with the sense capsules, does not long remain merely as a floor. Its sides grow vertically upwards, forming the =exoccipital= region of the cranium behind, and the =alisphenoidal= and =orbitosphenoidal= regions further forwards. In many forms, such as Elasmobranchs, all these upgrowths meet round the brain, roofing it in and forming an almost complete cartilaginous cranium. But in most vertebrata, while in the occipital region, the cartilaginous cranium is completed dorsally, in the alisphenoidal and orbitosphenoidal regions the cartilage merely forms the lateral walls of the cranium, the greater part of the brain having dorsal to it a wide space, closed by merely membranous tissue in connection with which the large frontal and parietal bones are subsequently formed. The SKULL includes _a._ the cranium, _b._ the jaws and visceral skeleton. The =cranium= can be further subdivided into (1) an axial portion, the =cranium proper= or =brain case=; (2) =the sense capsules.= The capsules of the auditory and olfactory sense organs are always present, and as has been already mentioned, in many animals the eye likewise is included in a cartilaginous capsule. (1) THE CRANIUM PROPER OR BRAIN CASE. The cranium varies much in form and structure. In lower vertebrates, such as Sharks and Lampreys, it remains entirely cartilaginous and membranous, retaining throughout life much of the character of the embryonic rudiment of the cranium of higher forms. The dogfish's cranium, described on pp. 73 to 76, is a good instance of a cranium of this type. But in the majority of vertebrates the cartilage becomes more or less replaced by cartilage bone, while membrane bones are also largely developed and supplant the cartilage. The cranium of most vertebrates includes a very large number of bones whose arrangement varies much, but one can distinguish a definite =basicranial axis= formed of the basi-occipital, basisphenoid, and presphenoid bones, which is a continuation forwards of the axis of the vertebral column. From the basicranial axis a wide arch arises, composed of a number of bones, which form the sides and roof of the brain-case These bones are arranged in such a manner that if both cartilage and membrane bones are included they can be divided into three rings or segments. The hinder one of these segments is the occipital, the middle the parietal, and the anterior one the frontal.