VII SKULL Zia 
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 Holocephah, 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. 135) 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. Relatively 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. 
