At the present day the Teleosteans are in immense preponderance, Selachians are still well represented, the Chondrostean ganoids are confined to the rivers and lakes of the temperate zone of the northern hemisphere (_Acipenseridae_, _Polyodontidae_), the Holostean ganoids are reduced to a few species (_Lepidosteus_, _Amia_) dwelling in the fresh waters of North America, Mexico and Cuba, the Crossopterygians are represented by the isolated group _Polypteridae_, widely different from any of the known fossil forms, with about ten species inhabiting the rivers and lakes of Africa, whilst the Dipneusti linger in Australia (_Neoceratodus_), in South America (_Lepidosiren_), and in tropical Africa (_Protopterus_). The imperfections of the geological record preclude any attempt to deal with the distribution in space as regards extinct forms, but several types, at present very restricted in their habitat, once had a very wide distribution. The _Ceratodontidae_, for instance, of which only one species is now living, confined to the rivers of Queensland, has left remains in Triassic, Rhaetic, Jurassic and Cretaceous rocks of Europe, North America, Patagonia, North and South Africa, India and Australia; the _Amiidae_ and _Lepidosteidae_ were abundant in Europe in Eocene and Miocene times; the _Osteoglossidae_, now living in Africa, S.E. Asia and South America, occurred in North America and Europe in the Eocene. In treating of the geographical distribution of modern fishes, it is necessary to distinguish between fresh-water and marine forms. It is, however, not easy to draw a line between these categories, as a large number of forms are able to accommodate themselves to either fresh or salt water, whilst some periodically migrate from the one into the other. On the whole, fishes may be roughly divided into the following categories:-- I. Marine fishes. A. shore-fishes; B. pelagic fishes; C. deep-sea fishes. II. Brackish-water fishes. III. Fresh-water fishes. IV. Migratory fishes. A. anadromous (ascending fresh waters to spawn); B. catadromous (descending to the sea to spawn). About two-thirds of the known recent fishes are marine. Such are nearly all the Selachians, and, among the Teleosteans, all the _Heteromi_, _Pediculati_ and the great majority of _Apodes_, _Thoracostei_, _Percesoces_, _Anacanthini_, _Acanthopterygii_ and _Plectognathi_. All the _Crossopterygii_, _Dipneusti_, _Opisthomi_, _Symbranchii_, and nearly all the _Ganoidei_ and _Ostariophysi_ are confined to fresh-water. The three categories of marine fishes have thus been defined by Günther:--