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. 3. _Galaxias attenuatus_ inhabits Tasmania, New Zealand, the Falkland Islands, and the southernmost part of the South American continent. 4. Several Petromyzonts enter the fresh waters of Tasmania, South Australia, New Zealand, and Chili. _B._ Genera identical in distant continents-- 1. The genus _Umbra_, so peculiar a form as to be the type of a distinct family consisting of two most closely allied species only, one of which is found in the Atlantic States of North America, the other in the system of the Danube. 2. A very distinct genus of Sturgeons, _Scaphirhynchus_, consisting of two species only, one inhabiting fresh waters of Central Asia, the other the system of the Mississippi. 3. A second most peculiar genus of Sturgeons, _Polyodon_, consists likewise of two species only, one inhabiting the Mississippi, the other the Yang-tse-kiang. 4. _Amiurus_, a Siluroid, and _Catostomus_, a Cyprinoid genus, both well represented in North America, occur in a single species in temperate China. 5. _Lepidosiren_ is represented by one species in tropical America, and by the second in tropical Africa (_Protopterus_). 6. _Notopterus_ consists of three Indian and two West African species. 7. _Mastacembelus_ and _Ophiocephalus_, genera characteristic of the Indian region, emerge severally by a single species in West and Central Africa. 8. _Symbranchus_ has two Indian and one South American species. 9. _Prototroctes_, the singular antarctic analogue of _Coregonus_, consists of two species, one in the south of Australia the other in New Zealand.