{232}[Illustration: 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 {233}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. [Illustration: 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.)]