If the distribution of the crayfishes is compared with that of terrestrial animals in general, the points of {314} difference are at least as remarkable as the resemblances.
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.
[35] According to Dr. Günther their southern range is similarly limited by the Asiatic Highlands. But they abound in the rivers both of the old and new worlds which flow into the Arctic sea; and though those on the western side of the Rocky Mountains are different from the Eastern American forms, yet there are species common to both the Asiatic and the American coasts of the North Pacific.
Thus, these fish avoid South Africa, as the crayfishes {316} do; but I am not aware that any member of the group is found in Madagascar, and thus completes the analogy.
The preservation of the soft parts of animals in the fossil state depends upon favourable conditions of rare occurrence; and, in the case of the Crustacea, it is not often that one can hope to meet with such small hard parts as the abdominal members, in a good state of preservation. But without recourse to the branchial apparatus, and to the abdominal appendages, it might be very difficult to say whether a given crustacean belonged to the Astacine, or to the closely allied Homarine group. Of course, if the accompanying fossils indicated that the deposit in which the remains occur, was of freshwater origin, the presumption in favour of their Astacine nature would be very strong; but if they were inhabitants of the sea, the problem whether the crustacean in question was a marine Astacine, or a true Homarine, might be very hard to solve.
Undoubted remains of crayfishes have hitherto been discovered only in freshwater strata of late tertiary age. In Idaho, North America, Professor Cope[36] found, in association with Mastodon mirificus, and Equus excelsus, several species, which he considers to be distinct from {317} the existing American crayfishes; whether they are Cambari or Astaci does not appear. But, in the lower chalk of Ochtrup, in Westphalia, and therefore in a marine deposit, Von der Marck and Schlüter[37] have obtained a single, somewhat imperfect, specimen of a crustacean, which they term Astacus politus, and which, singularly enough, has the divided telson found only in the genus Astacus. It would be very desirable to know more about this interesting fossil. For the present it affords a strong presumption that a marine Potamobine existed as far back as the earlier part of the cretaceous epoch.