The fishes included under the term Ganoidei form a very heterogeneous group, some of which closely approach the Dipnoi, others the Elasmobranchii, others the Teleostei. The great majority of them are extinct, only eight living genera being known; these are all inhabitants of the northern hemisphere, and with the exception of _Acipenser_, which is both fluviatile and marine, are entirely confined to fresh water. The following is a list of the living genera of Ganoids with their respective habitats:-- _Acipenser._ Rivers and seas of the northern hemisphere. _Scaphirhynchus._ Mississippi and rivers of Central Asia. _Polyodon_ (_Spatularia_). Mississippi. _Psephurus._ Yan-tse-kiang, and Hoangho. _Polypterus._ Rivers of tropical Africa. _Calamoichthys._ Some rivers of West Africa. _Lepidosteus._ Freshwaters of Central and North America and Cuba. _Amia._ Rivers of Carolina. The exoskeleton is very variable, thus the body may be:-- (_a_) Naked or with minute stellate ossifications as in the Polyodontidae. (_b_) Partially covered with large detached bony plates as in _Scaphirhynchus_ and _Acipenser_. (_c_) Entirely covered with rhomboidal ganoid scales as in _Lepidosteus_, _Polypterus_, _Palaeoniscus_ and many extinct forms. (_d_) Covered with rounded scales shaped like the cycloid scales of Teleosteans as in _Amia_. (_e_) Having the trunk and part of the tail covered with rhomboidal scales, and the remainder of the tail with rounded scales as in _Trissolepis_. 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_. _Suborder_ (1). CHONDROSTEI.