The substance of the skull of the Chondropterygians is cartilage, interrupted especially on its upper surface by more or less extensive fibro-membranous fontanelles. Superficially it is covered by a more or less thick chagreen-like osseous deposit. The articulation with the vertebral column is effected by a pair of lateral condyles. In the Sharks, besides, a central conical excavation corresponds to that of the centrum of the foremost vertebral segment, whilst in the Rays this central excavation of the skull receives a condyle of the axis of the spinous column.
The cranium itself is a continuous undivided cartilage, in which the limits of the orbit are well marked by an anterior and posterior protuberance. The ethmoidal region sends horizontal plates over the nasal sacs, the apertures of which retain their embryonic situation upon the under surface of the skull. In the majority of Chondropterygians these plates are conically produced, forming the base of the soft projecting snout; and in some forms, especially in the long-snouted Rays and the Saw-fishes (Pristis) this prolongation appears in the form of three or more tubiform rods.
As separate cartilages there are appended to the skull a suspensorium, a palatine, mandible, hyoid, and rudimentary maxillary elements.
The suspensorium is movably attached to the side of the skull. It generally consists of one piece only, but in some Rays of two. In the Rays it is articulated with the mandible only, their hyoid possessing a distinct point of attachment to the skull. In the Sharks the hyoid is suspended from the lower end of the suspensorium together with the mandible.
What is generally called the upper jaw of a Shark is, as Cuvier has already stated, not the maxillary, but palatine. It consists of two simple lateral halves, each of which articulates with the corresponding half of the lower jaw, which is formed by the simple representative of Meckel’s cartilage.
Some cartilages of various sizes are generally developed on each side of the palatine, and one on each side of the mandible. They are called labial cartilages, and seem to represent maxillary elements.
The hyoid consists generally of a pair of long and strong lateral pieces, and a single mesial piece. From the former cartilaginous filaments (representing branchiostegals) pass directly outwards. Branchial arches, varying in number, and similar to the hyoid, succeed it. They are suspended from the side of the foremost part of the spinous column, and, like the hyoid, bear a number of filaments.
The vertical fins are supported by interneural and interhæmal cartilages, each of which consists of two and more pieces, and to which the fin-rays are attached without articulation.
The scapular arch of the Sharks is formed by a single coracoid cartilage bent from the dorsal region downwards and forwards. In some genera (Scyllium, Squatina) a small separate scapular cartilage is attached to the dorsal extremities of the coracoid; but in none of the Elasmobranchs is the scapular arch suspended from the skull or vertebral column; it is merely sunk, and fixed in the substance of the muscles. Behind, at the point of its greatest curvature, three carpal cartilages are joined to the coracoid, which Gegenbaur has distinguished as propterygium, mesopterygium, and metapterygium, the former occupying the front, the latter the hind margin of the fin. Several more or less regular transverse series of styliform cartilages follow. They represent the phalanges, to which the horny filaments which are imbedded in the skin of the fin are attached.
In the Rays, with the exception of Torpedo, the scapular arch is intimately connected with the confluent anterior portion of the vertebral column. The anterior and posterior carpal cartilages are followed by a series of similar pieces, which extend like an arch forwards to the rostral portion of the skull, and backwards to the pubic region. Extremely numerous phalangeal elements, longest in the middle, are supported by the carpals, and form the skeleton of the lateral expansion of the so-called disk of the Ray’s body, which thus, in fact, is nothing but the enormously enlarged pectoral fin.