The paucity of freshwater fishes is due, in the first place, to the arid climate and the deficiency of water in the Australian continent, as well as to the insignificant size of the freshwater courses in the smaller islands. Still this cannot be the only cause: the large island of Celebes, which, by its mountainous portions, as well as by its extensive plains and lowlands, would seem to offer a favourable variety of conditions for the development of a freshwater Fauna is, as far as has been ascertained, tenanted by seven Freshwater fishes only, viz. 2 _Arius_, 2 _Plotosus_, 1 _Andbas_, 1 _Ophiocephalus_, 1 _Monopterus_, all of which are the commonest species of the Indian region. New Guinea has not yet been explored, but, from the faunæ nearest to this island, we expect its freshwater fishes will prove to be equally few in number, and identical with those of Celebes and North Australia; a supposition confirmed by the few small collections which have reached Europe. Finding, then, that even those parts of this region, which are favourable to the development of Freshwater fishes, have not produced any distinct forms, and that the few species which inhabit them, are unchanged, or but slightly modified Indian species, we must conclude that the whole of this area has remained geologically isolated from the other regions of this zone since the commencement of the existence of Teleostei; and that, with the exception of _Ceratodus_ and _Osteoglossum_, the immigration of the other species is of very recent date. Fossil remains of _Ceratodus_ have been found in Liassic and Triassic formations of North America, England, Germany, and India; and it is, therefore, a type which was widely spread in the Mesozoic epoch. Although it would be rash to conclude that its occupation of Australia dates equally far back, for it may have reached that continent long afterwards; yet it is evident that, as it is one of the most ancient of the existing types, so it is certainly the first of the Freshwater fishes which appeared in Australia. _Osteoglossum_, of which no fossil remains yet have been found, is proved by its distribution to be one of the oldest Teleosteous types. There must have been a long gap of time before these ancient types were joined by the other Teleostei. All of them migrated through the intervening parts of the ocean from India. Most of the _Plotosina_, some of the _Arii_, _Dules_, and the _Atherinichthys_, also _Nannoperca_ (allied to _Apogon_), were among the earliest arrivals, being sufficiently differentiated to be specifically or even generically (_Cnidoglanis_, _Nannoperca_) distinguished; but some others, like _Anabas scandens_, _Lates calcarifer_, _Dules marginatus_, must have reached the Australian continent quite recently, for they are indistinguishable from Indian specimens.