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 brasiliennis 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 re- 

 ferred to the genera Astacoides and Chwraps. The only Tasmanian 

 species which have been described constitute the genus Enyceus 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 of Mada- 

 gascar 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, nol to be accounted 

 for by auyapparent 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 escarp- 

 ment 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 fur- 

 nished by extant descriptions, of the actual amount of resemblance 

 and difference between the Crayfishes which are found in the dif- 

 ferent 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 distin- 

 guished are almost trivial. Erichson, however, has drawn attention 



pensis is neither a Crayfish nor a Lobster, and that, unless he was wrongly in- 

 formed, 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 the defini- 

 tion of the genus Homarus (t. c. p. 333) that the Lobsters " ne se trouvent que 

 dans la mer," and as Homarus has only three pairs of chelate limbs, the identi- 

 fication presents difficulties. Krauss (Siidafrikanisehe C'rustaceeu, p. 54), under 

 the head of "Homarus capensis," refers to Herbst and Milne-Edwards, 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. dejtressa and Homarus capensis'' as the only 

 South-African freshwater Thoracostraca. 



1 " Siidbrasilische Suss- und Brackwdsser-Orustaceen," ArcliiY fur >'atur- 

 gesehichte, 1869. 



