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.
Fig. 133.—Side view of the skull of Protopterus, with the pectoral girdle and fin. an, Angular; an.c, antorbital cartilage; c.c, coracoid cartilage (epi-coracoid); c.hy, cerato-hyal; cl, clavicle; c.r, cranial rib; c.sc, coraco-scapular cartilage; d.e, dermal ethmoid; d.l.e, dermal lateral ethmoid; e.g.f, external gills; eo, exoccipital; f.p, fronto-parietal; mk.c, Meckel's cartilage; n.a, neural arches; ol.c, fenestrated roof of the olfactory capsule; p.f, skeleton of the pectoral fin; p.pt, palato-pterygoid bone; p.q, palato-quadrate cartilage; s.cl, supra-clavicle; sp, splenial; sq, squamosal; 1-6, the branchial arches; the segmentation of the second and third arches is not shown. (From Wiedersheim.)
Relatively small opercular and inter-opercular bones are present, and on the inner surface of each may be found vestigial remains of cartilaginous hyoidean rays. The chondrocranium is complete in Neoceratodus, but in the remaining genera it has undergone considerable absorption in the inter-orbital region, so that the roof and floor, and, in part, even the side walls of the cranial cavity, are formed by the fronto-parietal and parasphenoid bones. Two exoccipitals are present in all Dipnoi. There are small labial cartilages in relation with the ventrally-placed nostrils, and large lateral outgrowths from the ethmoid cartilage furnish the olfactory organs with conspicuous lattice-like roofs. A pair of strong palato-pterygoid bones fringe the lower margins of the palato-quadrate cartilage, and meeting in front beneath the ethmoid region their symphysial extremities support the large palatal teeth. The Meckelian cartilages are persistent in all Dipnoi. In Neoceratodus each is flanked by a dentary and an angular externally, and internally by a splenial; but in Protopterus and Lepidosiren distinct dentary bones are wanting. The hyoid arch is best developed in Neoceratodus,[[206]] and includes a small hyomandibular cartilage, a partially bony cerato-hyal and cartilaginous hypo-hyal and basi-hyal element. In the other genera (Fig. 133) only a cerato-hyal is retained. The branchial arches are but feebly developed in the Dipnoi. Neoceratodus has five, of which the first four are divided into epi-branchial and cerato-branchial segments, while the fifth is undivided. Protopterus has six, but only the second and third are segmented as in Neoceratodus.[[207]] In Lepidosiren all the arches are simple undivided rods.
In all three genera the skull conforms to the same general type of structure, but it is much more primitive in Neoceratodus than in the other two genera.
Fig. 134.—Dorsal view of the skull of Lepidosiren. an.c, Condyle on the quadrate cartilage for the lower jaw; n.sp, neural spine; op, operculum. For other reference letters see Fig. 133. (From Bridge.)
With reference to the fossil Dipnoi, it may be stated that, so far as they are known, the cranial roofing bones are more numerous than in the existing genera, and they cannot readily be compared with those of the latter, or with the numerically reduced and more definitely arranged bones of most Teleostomi. There is also evidence that in some fossil Dipnoi (e.g. Dipterus) the chondrocranium and the mandibular suspensorium (palato-quadrate) must have been replaced by cartilage bones to an extent which has no parallel in any of the surviving types.[[208]] Jugular bones were present in Dipterus and Phaneropleuron.
Median Fins and Appendicular Skeleton