Order Actinistia.—In the Actinistia there is a single fin-ray to each basal bone, the axonosts of each ray fused in a single piece. The notochord is persistent, causing the back-bone in fossils to appear hollow, the cartilaginous material leaving no trace in the rocks. The genera and species are numerous, ranging from the Subcarboniferous to the Upper Cretaceous, many of them belonging to Cœlacanthus, the chief genus of the single family Cœlacanthidæ. In Cœlacanthus the fin-rays are without denticles. Cœlacanthus granulatus is found in the European Permian. Cœlacanthus elegans of the coal-measures is found in America also. In Undina the anterior fin-rays are marked with tubercles. Undina penicillata and Undina gulo from the Triassic are well-preserved species. In Macropoma (lewesiensis) the fin-rays are robust, long, and little articulated. Other genera are Heptanema, Coccoderma, Libys, Diplurus, and Graphiurus. Diplurus longicaudatus was found by Newberry in the Triassic of New Jersey and Connecticut.
Fig. 376.—Undina gulo Egerton; Lias. Family Cœlacanthidæ. (After Woodward.)
Order Cladistia.—In the Cladistia the axis of the pectoral limb is fan-shaped, made of two diversified bones joined by cartilage. The notochord is restricted and replaced by ossified vertebræ. The axonosts of the dorsal and anal are in regular series, each bearing a fin-ray. The order contains the single family Polypteridæ. In this group the pectoral fin is formed differently from that of the other Crossopterygians, being broad, its base of two diverging bones with cartilage between. This structure, more specialized than in any other of the Crossopterygians or Dipneusti, has been regarded by Gill and others, as above stated, as the origin of the fingered hand (chiropterygium) of the frogs and higher vertebrates. The base of the diverging bones has been identified as the antecedent of the humerus, the bones themselves as radius and ulna, while the intervening non-ossified cartilage breaks up into carpal bones, from which metacarpals and digits ultimately diverge. This hypothesis is open to considerable doubt. The nostrils, as in true fishes, are superior. The body in these fishes is covered with rhombic enameled scales, as in the garpike; the head is similarly mailed, but, in distinction from the garpike, the anterior rays of the dorsal are developed as isolated spines.
The young have a bushy external gill with a broad scaly base. The air-bladder is double, not cellular, with a large air-duct joining the ventral surface of the œsophagus. The intestine has a spiral valve.
The cranium, according to Boulenger ("Poissons du Bassin du Congo," p. 11), is remarkable for its generalized form, this character forming a trait of union between the Ganoids and the primitive Amphibia or Stegocephali. Without considering Polypterus, it is not possible to interpret the homologies of the cranium of the amphibians and the sharks.
The jaws are similar to those of the vertebrates higher than fishes. Tooth-bearing premaxillaries and dentaries are solidly joined at the front of the cranium, and united by a suture to the toothed maxillaries which form most of the edge of the mouth. Each half of the lower jaw consists of four elements, covering Meckel's cartilage, which is ossified at the symphysis. These are the articular, angular, dentary, and splenial (coronoid). Most of these bones are armed with teeth. The palato-suspensory consists of hyomandibular, quadrate, ectopterygoid, entopterygoid, metapterygoid, and palatine elements, the pterygoid elements bearing teeth. In Erpetoichthys only the opercle is distinct among the gill-covers. In Polypterus there is a subopercle also; the suborbital chain is represented by two small bones.
The gill-arches are four, but without lower pharyngeals. The teeth are conic and pointed, and in structure, according to Agassiz, they differ largely from those of bony fishes, approaching the teeth of reptiles.
Fig. 377.—Lower jaw of Polypterus bichir, from below.