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. [Illustration 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. 2. orbit. 3. parasphenoid. 4. vomer. 5. pterygoid. 6. maxilla. (The dotted line running from 6 passes into the mouth cavity.) 7. dentary. 8. symplectic. 10. palatine. 11. hyomandibular. 12. pharyngo-branchial. 13. epi-branchial. 14. cerato-branchial. 15. hypo-branchial. 16. coalesced anterior vertebrae. 17. inter-hyal. 18. cerato-hyal. 19. rib.]