An interesting modification of certain of the bones of the primary and secondary upper jaw occurs in the Siluridae. In these Fishes the maxillae are very small and edentulous, and serve no other purpose than forming basal supports for the maxillary barbels, while the rod-like palatine bone, losing its connexion with the pterygoid portion of the primitive upper jaw, {231}but retaining its articulation with the lateral ethmoid, serves to support the maxilla, and at the same time receives the insertion of the muscles by which the barbel is moved in various directions. In the Plectognathi the premaxillae are co-ossified with the maxillae. Many other interesting cranial modifications occur in Teleosts, and to some of them reference is made in subsequent chapters. In some respects the skull of Dipnoi[205] is remarkably like that of the Holocephali, especially in its typical autostylism; but in possessing both cartilage- and membrane-bones it in some measure approaches the Teleostome skull. The investing dermal bones are not always easy to identify with those of other Fishes. In _Neoceratodus_ an anterior median membrane-bone or dermal mesethmoid covers the ethmo-nasal region, and, on each side of it, forming the anterior boundary of the orbit, there is situated a pre-orbital or dermal lateral ethmoid. Behind the mesethmoid there is a much larger posterior median bone, and on each side a singular backward prolongation of the dermal lateral ethmoid separates it from a squamosal element. The latter bone descends on the outer surface of the quadrate portion of the palato-quadrate cartilage as far as the condyle for the lower jaw. Collectively, these bones form a fairly complete investment to the upper surface of the cranium, but the posterior median bone and the adjacent portions of the dermal lateral ethmoid and the squamosal are widely separated from the underlying chondrocranium by the powerful jaw muscles, and in this respect they differ from the ordinary roofing bones of other Fishes. In _Protopterus_ (Fig. 133) and _Lepidosiren_ (Fig. 134) the posterior median bone is non-existent, and its place is taken by a large fronto-parietal, which forms the greater part of the cranial roof, internal to the jaw muscles, and is much larger in the latter Dipnoid than in the former. Circum-orbital bones are present only in _Neoceratodus_. A large parasphenoid supports the cranial floor. Vomers are absent, although there are two small vomerine teeth.