With respect to the latter, the area occupied by the _Potamobiidæ_, corresponds roughly with the Palæarctic and Nearctic divisions of the great Arctogæal provinces of distribution indicated by mammals and birds; while distinct groups of crayfishes occupy a larger or smaller part of the other, namely, the Austro-Columbian, Australian, and Novozelanian primary distributional provinces of mammals and birds. Again, the peculiar crayfishes of Madagascar answer to the special features of the rest of the fauna of that island. But the North American crayfishes extend much further South than the limits of the Nearctic fauna in general; while the absence of any group of crayfishes in Africa, or in the rest of the old world, south of the great Asiatic table-land, forms a strong contrast to the general resemblance of the North African and Indian fauna to that of the rest of Arctogæa. Again, there is no such vast difference between the crayfishes of New Zealand, Australia, and South America, as there is between the mammals and the birds of those regions. It may be concluded, therefore, that the conditions which have determined the distribution of crayfishes have been very different from those which have governed the distribution of mammals and birds. But if we compare with the distribution of the crayfishes, not that of terrestrial animals in general, but only that of freshwater {315} fishes, some very curious points of approximation become manifest. The _Salmonidæ_, or fishes of the salmon and trout kind, a few of which are exclusively marine, many both marine and freshwater, while others are confined to fresh water, are distributed over the northern hemisphere, in a manner which recalls the distribution of the Potamobine crayfishes,[35] though they do not extend so far to the South in the new world, while they go a little further, namely, as far as Algeria, Northern Asia Minor, and Armenia, in the old world. With the exception of the single genus _Retropinna_, which inhabits New Zealand, no true salmonoid fish occurs south of the equator; but, as Dr. Günther has pointed out, two groups of freshwater fishes, the _Haplochitonidæ_ and the _Galaxidæ_, which stand in somewhat the same relation to the _Salmonidæ_ as the _Parastacidæ_ do to the _Potamobiidæ_, take the place of the _Salmonidæ_ in the fresh waters of New Zealand, Australia, and South America. There are two species of _Haplochiton_ in Tierra del Fuego; and of the closely allied genus _Prototroctes_, one species is found in South Australia, and one in New Zealand; of the _Galaxidæ_, the same species, _Galaxias attennuatus_, occurs in the streams of New Zealand, Tasmania, the Falkland Islands, and Peru.