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. |
As in every other class of animals, these freshwater genera and families vary greatly with regard to the extent of their geographical range; some extend over the greater half of the continental areas, whilst others are limited to one continent only, or even to a very small portion of it. As a general rule, a genus or family of freshwater fishes is regularly dispersed and most developed within a certain district, the species and individuals becoming scarcer towards the periphery as the type recedes more from its central home, some outposts being frequently pushed far beyond the outskirts of the area occupied by it. But there are not wanting those remarkable instances of closely allied forms occurring, almost isolated, at most distant points, without being connected by allied species in the intervening space; or of members of the same family, genus, or species inhabiting the opposite shores of an ocean, and separated by many degrees of abyssal depths. We mention of a multitude of such instances the following only:—
A. Species identical in distant continents—
1. A number of species inhabiting Europe and the temperate parts of eastern North America, as Perca fluviatilis, Gastrosteus pungitius, Lota vulgaris, Salmo solar, Esox lucius, Acipenser sturio, Acipenser maculosus, and several Petromyzonts.
2. Lates calcarifer is common in India as well as in Queensland.