3. The branchial arches, generally five in number, all of which except the last bear branchiostegal rays. In the Notidanidae the number of branchial arches is increased beyond the normal series, thus in Hexanchus there are six, and in Heptanchus seven. There are six also in Chlamydoselache and Protopterus.

4. The so-called external branchial arches which are cartilaginous rods attached to all the visceral arches. They are especially large in Cestracion.

The skull in Holocephali is entirely cartilaginous. The palato-pterygo-quadrate bar is fixed to the cranium, and to it the mandible articulates. There is a well-marked joint between the skull and the spinal column.

In living Cartilaginous Ganoids the primitive cartilaginous cranium is very massive, and is greatly prolonged anteriorly, while posteriorly it merges into the spinal column. Although it is mainly cartilaginous a number of ossifications take place in the skull, and membrane bones are now found definitely developed, especially in connection with the roof of the cranium. In Acipenser (fig. 18) the ossifications in the cartilage include the pro-otic, which is pierced by the foramen for the fifth nerve, the alisphenoid, orbitosphenoid, ectethmoid, palatine, pterygoid, meso-pterygoid, hyomandibular (fig. 18, 11), cerato-hyal, all the cerato-branchials, and the first two epi-branchials. Most of these structures are, however, partly cartilaginous, though they include an ossified area. The membrane bones too of Acipenser are very well developed, they include a bone occupying the position of the supra-occipital, and form a complete dorsal cephalic shield. Resting on the ventral surface are a vomer and a very large parasphenoid (fig. 18, 3). There is a bony operculum attached to the hyomandibular, and membrane bones representing respectively the maxilla and dentary are attached to the jaws. The suspensorium is most markedly hyostylic. The palato-pterygo-quadrate bar has a very curious shape and is quite separate from the cranium. It is connected to the hyomandibular by a thick symplectic ligament containing a small bone homologous with the symplectic of Teleosteans.

Polyodon differs much from Acipenser, the membrane bones not being so well developed though they cover the great cartilaginous snout.

Fig. 18. Lateral view of the skull of a Sturgeon (Acipenser sturio). Nearly all the membrane bones have been removed (Brit. Mus.).

1. nasal cavity.10. palatine.
2. orbit.11. hyomandibular.
3. parasphenoid.12. pharyngo-branchial.
4. vomer.13. epi-branchial.
5. pterygoid.14. cerato-branchial.
6. maxilla. (The dotted line15. hypo-branchial.
running from 6 passes16. coalesced anterior vertebrae.
into the mouth cavity.)17. inter-hyal.
7. dentary.18. cerato-hyal.
8. symplectic.19. rib.

The skull in Polypterus (Crossopterygii) shows a great advance towards the condition met with in Teleostei. The cranium remains to a great extent unossified, and large dorsal and ventral fontanelles pierce its walls. It is covered by a great development of membrane bones, paired nasals, frontals, parietals, supra- and post-temporals, and dermo-supra-occipitals among others being present. The palato-pterygo-quadrate bar is fused to the cranium, and in connection with it the following paired membrane bones appear, palatine, ecto-, meso- and meta-pterygoid, and further forwards jugal, vomer, maxilla and premaxillae. The membrane bones developed in connection with each ramus of the mandible are the dentary, angular, and splenial, in addition to the cartilage bone the articular. Several large opercular bones occur. There are also a pair of large jugular or gular plates, and several large opercular bones.

In Bony Ganoids both cartilage bone and membrane bone is well developed. The pro-otics and exoccipitals are well ossified, but the supra-occipital and pterotics are not. Lateral ethmoids are developed, and there are ossifications in the sphenoidal region which vary in different forms. The place of the cartilaginous palato-pterygo-quadrate is taken by a series of bones, the quadrate behind and the palatine, ecto-, meso- and meta-pterygoids in front. In Lepidosteus, however, the palatine and pterygoid are membrane bones, as they are in Polypterus and the Frog. Paired maxillae, premaxillae, vomers and a parasphenoid occur forming the upper jaw and roof of the mouth, and a series of membrane bones are found investing the mandible and forming the operculum.