vni SKULL 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 : 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 invest- 

 ment 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 Neoceralodus. A large parasphenoid supports 

 the cranial floor. Vomers are absent, although there are two 

 small vomerine teeth. Ilelatively small opercular and inter- 

 opercular bones are present, and on the inner surface of each 



1 Giinther, Phil. Trans. 161, 1871, p. 521 ; Huxley, P.Z.S. 1876, p. 31 ; 

 Wiedersheim, Morph. Stud. i. Jena, 1880, p. 46 ; Bridge, Trans. Zool. Soc. xiv. 

 1898, p. 350. 



