The group includes the vast majority of living fish (see p. 33).
Order V. Dipnoi.
The exoskeleton is of two types; dermal bones are largely developed in the head region, while the tail and posterior part of the body may be naked or may be covered with overlapping scales. The cranium remains chiefly cartilaginous, the palato-pterygo-quadrate bar is fused with the cranium, and the suspensorium is autostylic. The gill clefts are feebly developed and open into a cavity covered by an operculum. The notochord is persistent and unconstricted, and the limbs are archipterygia. The pelvic fins are without claspers.
Suborder (1). Sirenoidei[33].
The head has well developed membrane bones. The trunk is covered with overlapping scales and bears no bony plates. Three pairs of teeth are present, two in the upper and one in the lower jaw, the two principal pairs of teeth are borne on the palato-pterygoids and splenials, while the third pair are found in the vomerine region. The tail is diphycercal in living forms. In the extinct Dipteridae it is heterocercal. The pectoral girdle includes both membrane and cartilage bones. The pelvic girdle consists of a single bilaterally symmetrical piece of cartilage.
This suborder is represented by the living genera Ceratodus, Protopterus and Lepidosiren, and among extinct forms by the Dipteridae and others.
Suborder (2). Arthrodira.
Bony plates are developed not only on the head but also on the anterior part of the trunk, where they consist of a dorsal, a ventral, and a pair of lateral plates which articulate with the cranial shield. The posterior part of the trunk is naked. The tail is diphycercal. The jaws are shear-like, and their margins are usually provided with pointed teeth whose bases fuse with the tissue of the jaw and constitute dental plates. There seem to have been three pairs of these plates, arranged as in the Sirenoidei, the principal ones in the upper jaw being borne on the palato-pterygoids. Small pelvic fins are present, but pectoral fins are unknown.
The Arthrodira occur chiefly in beds of Devonian and Carboniferous age. Two of the best known genera are Coccosteus from the European Devonian and Dinichthys, a large predatory form from the lower Carboniferous of Ohio.