The simplest type of cranium is that found in Elasmobranchs: it consists of a simple cartilaginous box, which is generally immovably fixed to the vertebral column, though in some forms, like Scymnus and Galeus, a joint is indicated, and in others, such as the Rays, one is fairly well developed. The cranium in Elasmobranchs is never bony, though the cartilage is sometimes calcified. It is drawn out laterally into an antorbital process in front of the eye, and a postorbital process behind it. The nasal capsules are always cartilaginous, and the eye, as a general rule, has a cartilaginous sclerotic investment. The cranium is often prolonged in front into a rostrum which is enormously developed in Pristis and some Rays. The cartilaginous roof of the cranium is rendered incomplete by the presence of a large hole, the anterior fontanelle.

Two pairs of labial cartilages (fig. 16, B, 8) are often present. They lie imbedded in the cheeks outside the anterior region of the jaws, and are specially large in Squatina.

As regards the visceral arches[44] the simplest and most primitive condition of the jaws is that of the Notidanidae, in which the mandibular and hyoid arches are entirely separate. In these primitive fishes the palato-pterygo-quadrate bar articulates with the postorbital process (fig. 16, 10), while further forwards it is united to the cranium by the ethmo-palatine ligament. The hyoid arch is small and is broadly overlapped by the mandibular arch. The term autostylic is used to describe this condition of the suspensorium. From this condition we pass in the one direction to that of Cestracion (fig. 16, B), in which the whole of the palato-pterygo-quadrate bar has become bound to the cranium, and in the other to that of Scyllium. In Scyllium (fig. 6), while the ethmo-palatine ligament is retained, the postorbital articulation of the palato-pterygo-quadrate has been given up, so that the palato-pterygo-quadrate comes to abut on the hyomandibular and is attached to it by ligaments. The pre-spiracular ligament (fig. 16, 20) running from the auditory capsule also assists in supporting the jaws.

Lastly we come to the purely hyostylic condition met with in Rays, in which the mandibular arch is entirely supported by the hyomandibular. In some Rays the hyoid is attached to the posterior face of the hyomandibular near its proximal end, and may even come to articulate with the cranium.

The visceral arches of Elasmobranchs may be summarised as follows:—

1. The mandibular arch, consisting of a much reduced dorsal portion, the pre-spiracular ligament, and a greatly developed ventral portion from which both upper and lower jaws are derived. The mandible (Meckel's cartilage) is the original lower member of the mandibular arch, and from it arises an outgrowth which forms the upper jaw or palato-pterygo-quadrate bar. In Scymnus this bears a few branchiostegal rays.

2. The hyoid arch, which consists of the hyomandibular and the hyoid, and bears branchiostegal rays on its posterior face.

Fig. 17. Dorsal view of the Branchial arches of Heptanchus. (From Gegenbaur).

1. basi-hyal.7. pharyngo-branchial, common
2. cerato-hyal.to the sixth and seventh
3. second hypo-branchial.arches.
4. first cerato-branchial.8. basibranchial of second arch.
5. first epi-branchial.9. basibranchial, common to the
6. first pharyngo-branchial.sixth and seventh arches.