=The Skeleton of Sharks.=--In the Elasmobranchs (sharks, rays, chimæras) the tissues surrounding the notochord are segmented and in most forms distinct vertebræ are developed. Each of these has a conical cavity before and behind, with a central canal through which the notochord is continued. The form and degree of ossification of these vertebræ differ materially in the different groups. The skull in all these fishes is cartilaginous, forming a continuous undivided box containing the brain and lodging the organs of sense. To the skull in the shark is attached a suspensorium of one or two pieces supporting the mandible and the hyoid structures. In the chimæra the mandible is articulated directly with the skull, the hyomandibular and quadrate elements being fused with the cranium. The skull in such case is said to be _autostylic_, that is, with self-attached mandible. In the shark it is said to be _hyostylic_, the hyomandibular intervening. The upper jaw in the shark consists not of maxillary and premaxillary but of palatine elements, and the two halves of the lower jaw are representatives of Meckel's cartilage, which is the cartilaginous centre of the dentary bone in the bony fishes. These jaw-bones in the higher fishes are in the nature of membrane bones, and in the sharks and their relatives all such bones are undeveloped. The hyoid structures are in the shark relatively simple, as are also the gill-arches, which vary in number. The vertical fins are supported by interneural and interhæmal cartilages, to which the soft fin-rays are attached without articulation. [Illustration: FIG. 45.--Pectoral fin of _Heterodontus philippi_. (From nature.)] [Illustration: FIG. 46.--Pectoral fin of _Heptranchias indicus_ (Gmelin). (After Dean.)] The shoulder-girdle is made of a single cartilage, touching the back-bone at a distance behind the head. To this cartilage three smaller ones are attached, forming the base of the pectoral fin. These are called _mesopterygium_, _propterygium_, and _metapterygium_, the first named being in the middle and more distinctly basal. These three segments are subject to much variation. Sometimes one of them is wanting; sometimes two are grown together. Behind these the fin-rays are attached. In most of the skates the shoulder-girdle is more closely connected with the anterior vertebræ, which are more or less fused together. [Illustration: FIG. 47.--Shoulder-girdle of a Flounder, _Paralichthys californicus_ (Ayres).]