of the most characteristic forms of the marine and freshwater series are found, on taking a broader view of the subject, to be sufficiently distinct, but also because their distribution depends on causes different in their nature as well as the degree of their action, it will be necessary to treat of the two series separately. Whether the oceanic areas correspond in any way to the terrestrial will be seen in the sequel. [Illustration: Fig. 103.--Ganoid scales of _Tetragonolepis_.] CHAPTER XVII. THE DISTRIBUTION OF FRESHWATER FISHES. Having shown above that numerous marine fishes enter fresh waters, and that some of them have permanently established themselves therein, we have to eliminate from the category of freshwater fishes all such adventitious elements. They are derived from forms, the distribution of which is regulated by other agencies, and which, therefore, would obscure the relations of the faunæ of terrestrial regions if they were included in them. They will be mentioned with greater propriety along with the fishes constituting the fauna of the brackish water. True freshwater fishes are the following families and groups only:-- Dipnoi with 4 species. Acipenseridæ and Polyodontidæ „ 26 „ Amiidæ „ 1 „ Polypteridæ. „ 2 „ Lepidosteidæ. „ 3 „ Percina „ 46 „ Grystina „ 11 „ Aphredoderidæ „ 1 „ Centrarchina „ 26 „ Dules „ 10 „ Nandidæ „ 7 „ Polycentridæ „ 3 „ Labyrinthici „ 30 „ Luciocephalidæ „ 1 „ Gastrosteus „ 10 „ Ophiocephalidæ „ 31 „ Mastacembelidaæ „ 13 „ Chromides „ 105 „ Comephoridæ „ 1 „ Gadopsidæ „ 1 „ Siluridæ „ 572 „ Characinidæ „ 261 „ Haplochitonidæ „ 3 „ Salmonidæ (3 genera excepted) „ 135 „ Percopsidæ „ 1 „ Galaxiidæ „ 15 „ Mormyridæ (and Gymnarchidæ) „ 52 „ Esocidæ „ 8 „ Umbridæ „ 2 „ Cyprinodontidæ „ 112 „ Heteropygii „ 2 „ Cyprinidæ „ 724 „ Kneriidæ „ 2 „ Hyodontidæ „ 1 „ Osteoglossidæ „ 5 „ Notopteridæ „ 5 „ Gymnotidæ „ 20 „ Symbranchidæ „ 5 „ Petromyzontidæ „ 12 „ -------------- Total 2269 species. ==============