The teeth also are very variable. The endoskeleton shows every stage of transition from an almost entirely cartilaginous state as in _Acipenser_ to a purely bony state as in _Lepidosteus_. Sometimes, as in _Acipenser_, the notochord persists, and its sheath is unsegmented; sometimes, as in _Lepidosteus_, there are fully formed vertebrae. The tail may be heterocercal, as in _Acipenser_, or diphycercal as in _Polypterus_. The cartilaginous cranium is always covered with external membrane bone to a greater or less extent, and the suspensorium is markedly hyostylic. The pectoral girdle is formed of two parts, one endoskeletal and cartilaginous, corresponding with the pectoral girdle of Elasmobranchs, and one exoskeletal and formed of membrane bones, corresponding with the clavicular bones of Teleosteans. The pelvic fins are always abdominal. The fins often, as in _Polypterus_, have spines (fulcra) attached to their anterior borders.
The order Ganoidei may be divided into three suborders:
(1) CHONDROSTEI. Living genera _Acipenser_, _Scaphirhynchus_, _Polyodon_ and _Psephurus_.
(2) CROSSOPTERYGII. Living genera _Polypterus_ and _Calamoichthys_.
(3) HOLOSTEI. Living genera _Lepidosteus_ and _Amia_.
The skull is immovably fixed to the vertebral column. By far the greater part of the skeleton is cartilaginous. The notochord is persistent and unconstricted, its sheath is membranous, but cartilaginous neural and haemal arches are developed. Intercalary pieces (interdorsalia) occur between the neural arches, and similar pieces (interventralia) between the haemal arches. The cranium is covered with membrane bone, and teeth are but slightly developed. The tail is heterocercal. Gill rays occur on the hyoid arch, and the gills are protected by a bony operculum attached to the hyomandibular. The skin (1) may be almost or quite naked, (2) may carry bony plates arranged in rows, or may be covered (3) with rhomboidal scales, or (4) partly with rhomboidal, partly with cycloidal scales.
The exoskeleton has the form of cycloidal or rhomboidal scales. The condition of the vertebral column differs in the different genera. Sometimes, as in _Polypterus_, there are well-developed ossified vertebrae; sometimes, as in many extinct forms, the notochord persists and is unconstricted. The tail may be diphycercal or heterocercal. The pectoral and sometimes the pelvic fins consist of an endoskeletal axis bearing a fringe of dermal rays.