The skull of TELEOSTEI is very similar to those of _Lepidosteus_ and _Amia_. Although the bony skull is greatly developed and very complicated, much of the original cartilaginous cranium often persists. Membrane bones are specially developed on the roof of the skull where they include the parietal, frontal, and nasal bones. The same bones are developed in connection with the upper jaw and roof of the mouth as in bony Ganoids, but only two membrane bones occur in the lower jaw, viz. the angular and dentary. A number of large ossifications take place in the cartilage of the auditory capsules. In some forms parts of the last pair of branchial arches are broadened out and form the pharyngeal bones which bear teeth. The opercular bones and those of the upper and lower jaws are quite comparable to those of bony Ganoids. A full account of the Teleostean skull has been given in the case of the Salmon (pp. 87-96) and the Cod (pp. 96-101). In DIPNOI the skull is mainly cartilaginous, but both cartilage- and membrane-bone occur also. Cartilage-bone is found in the ossified exoccipitals, while of membrane-bones _Protopterus_ has among unpaired bones a fronto-parietal, a median ethmoid, and a parasphenoid, and among paired bones nasals and large supra-orbitals. The skull of _Ceratodus_ (fig. 19) has an almost complete roof of membrane bones, including some whose homology is doubtful. The ethmo-vomerine region is always cartilaginous, but bears small teeth. The palato-pterygo quadrate bar is ossified and firmly united to the cranium, and the mandible articulates directly with it (autostylic). Membrane bones are freely developed in connection with the mandible, dentary, splenial, and angular bones being all present. There are two opercular bones. In the extinct Dipteridae the cranium is very completely covered with plates of dermal bone, and the skeleton in general is more ossified than is the case in recent Dipnoi. Six pairs of branchial arches occur in _Protopterus_; _Ceratodus_ and _Lepidosiren_ have five, like most other fish. The branchial arches bear gill rakers. [Illustration FIG. 19. DORSAL (TO THE LEFT) AND VENTRAL (TO THE RIGHT) VIEWS OF THE CRANIUM OF _Ceratodus miolepis_ (after GÜNTHER). 1. cartilaginous part of the quadrate with which the mandible articulates. 2. scleroparietal. 3. frontal. 4. ethmoid. 5. nares. 6. orbit. 7. pre-opercular (squamosal). 8. second rib. 9. first rib. 10. vomerine tooth. 11. palato-pterygoid tooth. 12. palato-pterygoid. 13. parasphenoid. 14. interopercular.] RIBS. As has been already mentioned (p. 24), although ribs commonly appear to be the cut-off ends of the transverse processes, they are really elements derived from the ventral or haemal arch. In Elasmobranchii and other cartilaginous fish they have the form of small cartilaginous structures imperfectly separated from the diverging halves of the ventral arch, and are often absent.