FRESHWATER CRUSTACEA OF TASMANIA. 67 



Australia {Astacopsis and Mi g cbus), South America {Parastacus), and an isolated form in 

 Madagascar (Astacoides) . 



There is, therefore, a strong presumption in favour of the view that at any rate these 

 elements in the temperate Crustacean fauna of Southern Australasia have reached their 

 present range by means of an Antarctic connection between the southernmost projections 

 of Australia, S. America, and New Zealand — a connection which is now more than ever 

 postulated to account for the presence of common elements in the temperate fauna and 

 flora of these countries. 



Starting with this strong presumption in favour of an antarctic derivation of certain of 

 the Tasmanian and Southern Australian Crustacea, it is interesting to examine another 

 element which has evidently been derived primarily from the Northern Hemisphere. 

 This element consists of the Amphipodan genera Gammarus and Neoniphargus, the 

 Cladoceran Bosmina, and probably the Anaspidacea. 



The genus Neoniphargus, the members of which form so dominant a feature in the 

 Crustacean fauna of Tasmania, is very closely related to Gammarus, but it shows an 

 approach to the subterranean European genus Niphargus, especially in the form of the 

 first maxilla (PI. 13. fig. 5), the inner plate of which is narrow and bears only two or 

 three plumed setae at its apex, in the small number of joints composing the secondary 

 appendage of the first antenna (PI. 13. fig. 2), and in the greatly reduced endopodites of 

 the last pair of uropods (PI. 13. fig. 13). 



Undoubted representatives of the genus Gammarus also occur in Tasmania (e. g. 

 G. australis), and certain species (e. g. G. ripensis and G. antipodeus) are almost exactly 

 intermediate between the genera Neoniphargus and Gammarus (see PI. 14. fig. 27) in 

 the structure of the maxilla and of the first antenna (PL 14. figs. 17, 22, 23). The 

 special resemblance of Neoniphargus to the European Niphargus may therefore be due 

 to convergence. However this may be, the occurrence of Gammarus and the closely 

 related Neoniphargus in the temperate region of Southern Australasia and in New 

 Zealand affords a peculiar instance of discontinuous distribution, because Gammarus 

 and its close allies are otherwise confined to the north temperate hemisphere, being 

 absolutely unknown from the tropics of either hemisphere. In enquiring how they 

 have reached their present position in Southern Austi'alasia we must bear in mind that 

 temperature in this case, as in the case of all Crustacea, whether freshwater or marine, 

 is the chief condition determining the dispersal of these forms. There are two routes by 

 which they could have reached Southern Australasia, either through the tropics of Asia 

 and Northern Australia or else through South America and the lost Antarctic 

 connection. It is extremely difficult to see how an animal apparently incapable of 

 living in a tropical climate could have followed the first-named route, as there is no 

 range of mountains in the Eastern Hemisphere running north and south which could be 

 used by such an animal as a bridge to pass through the tropics ; but in America there 

 is the range of the Andes to permit this migration. It may therefore be suggested that 

 the Tasmanian and Southern Australian Gammarus and Neoniphargus reached these 

 countries from the Northern Hemisphere through S. America and Antarctica, and it is 

 reasonable to suppose that some of their representatives will still be found in the Andes. 



