A large operculum, and a smaller sub- or interoperculum are present. The _scapular_ arch consists of a single median transverse cartilage, and a pair of lateral cartilages which bear the articular condyle for the pectoral limb. The latter cartilages form the base of a large membrane-bone, and the whole arch is suspended from the skull by means of an osseous supraclavicle. The fore-limb of the _Dipnoi_ (Fig. 36) differs externally greatly from the pectoral fin of other Ganoid fishes. It is covered with small scales along the middle, from the root to its extremity, and surrounded by a rayed fringe similar to the vertical fin. A muscle split into numerous fascicles extends all the length of the fin, which is flexible in every part and in every direction. The cartilaginous framework supporting it is joined to the scapular arch by an oblong cartilage, followed by a broad basal cartilage (_a_), generally single, sometimes showing traces of a triple division. Along the middle of the fin runs a jointed axis (_b_), the joints gradually becoming smaller and thinner towards the extremity; each joint bears on each side a three, two, or one-jointed branch (_c, d_). This _axial_ arrangement of the pectoral skeleton, which evidently represents one of its first and lowest conditions, has been termed _Archipterygium_ by Gegenbaur. It is found in _Ceratodus_ and other genera, but in _Lepidosiren_ the jointed axis only has been preserved, with the addition of rudimentary rays in _Protopterus_. [Illustration: Fig. 36.--Fore-limb of Ceratodus.] The _pubic_ consists of a single flattened subquadrangular cartilage, produced into a long single anterior process. Posteriorly it terminates on each side in a condyle, to which the basal cartilage of the ventral paddle is joined. The endoskeleton of the paddle is almost identical with that of the pectoral. The Ganoid fishes with persistent notochord, but with a _hyostylic_ skull (that is, a skull with a separate suspensorium) consist of the suborder _Chondrostei_, of which the existing representatives are the Sturgeons (_Acipenser_, _Scaphirhynchus_, _Polyodon_), and the extinct the _Chondrosteidæ_, _Palæoniscidæ_, and (according to Traquair) _Platysomidæ_. Their spinal column does not differ essentially from that of the Dipnoi. Segmentation is represented only as far as the neural and hæmal elements are concerned. All are eminently heterocercal. Ribs are present in most, but replaced by ligaments in _Polyodon_. [Illustration: Fig. 37.--Skull of Polyodon (after Traquair). _n_, Nasal cavity; _sq_, squamosal; _mh_, hyomandibular; _sy_, symplectic; _pa_, palato-pterygoid; _m_, Meckelian cartilage; _mx_, maxillary; _d_, dentary; _h_, hyoid; _op_, opercle; _br_, branchiostegal; _s.cl_, supra-clavicular; _p.cl_, post-clavicular; _cl_, clavicle; _i.cl_, infra-clavicular.]