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Show 1878.] AND DISTRIBUTION OF THE CRAYFISHES. 755 Amazons, could find none. Two species from Southern Brazil have been described by Dr. von Martens ' as Astacus brasiliensis and A. pilimanus; but Von Martens recognizes the affinity of these forms with the Astacoides of Erichson. Several species of Paranephrops have been described from New Zealand; and the Fijian Crayfish belongs to the same genus. Crayfishes occur all over Australia ; and the species have been referred to the genera Astacoides and Chovraps. The only Tasmanian species which have been described constitute the genus Engceus of Erichson. Thus it appears, from what is already published on this subject: - 1. That the Crayfishes of the northern hemisphere are generically distinct from those of the southern hemisphere. 2. That the American Crayfishes, east of the Sierra Nevada, are generically distinct from those west of that range, as well as from the South-American species ; and that.while the western North-American Crayfishes belong to the same genus as those of the Old World, the South-American forms are more closely allied with those ot Madagascar and Australia. 3. That the New-Zealand species are distinct from the Australian forms ; and that the latter are to be placed in the same genus as the Madagascar and South-American species. 4. That there is a negative fact of distribution, not to be accounted for by anyapparent difference of climate or other physical conditions- namely, the entire absence of Crayfishes in Equatorial South America, Africa, and the rest of the Old World south of the northern escarpment of the great Asiatic highlands. The problem thus offered is one of the most remarkable among the many presented by the facts of Geographical Distribution ; and it appeared to me that one of the first steps towards attempting its solution was to obtain some more definite conception, than is furnished by extant descriptions, of the actual amount of resemblance and difference between the Crayfishes which are found in the different areas of distribution. For the most part the Crayfishes are so similar in their general structure, that the characters by which the genera have been distinguished are almost trivial. Erichson, however, has drawn attention pernis is neither a Crayfish nor a Lobster, and that, unless he was wrongly informed, it is an inhabitant of fresh water. Milne-Edwards (Hist. Nat. des Crustaces, ii. p. 335) identifies his Homarus capensis with the Cancer capensis of Herbst; but, as it is stated in tbe definition of the genus Homarus (I. c. p. 333) that the Lobsters " ne se trouvent que dans la mer," and as Homarus has only three pairs of chelate limbs, the identification presents difficulties. Krauss (Siidaf rikanische Crustaceen, p. 54), under the head of "Homarus capensis," refers to Herbst and Milne-Edwai-ds, and, apparently on the authority of the former, merely says:-" In den Bergflussen des Kaplandes. Ich habe ihn in Natal nie gesehen." Elsewhere (p. 20) he gives " Thelphusa perlata and T. depressa and Homarus capensis" as the only South-African freshwater Thoracostraca. 1 " Sudbrasilische Suss- und Brackwasser-Cru3taceen," Archir fur Natur-geschichte, 1869. |
