CRUSTACEA OF FRESH WATERS 177 



ever, not so easy to understand. The species found 

 in Europe all belong to the genus Astacus, which 

 also penetrates into Asia as far as Turkestan and the 

 basin of the River Obi. 



Throughout the greater part of Asia no Crayfishes 

 are found until we come to the Far East, where we 

 find an isolated colony in the river-system of the Amur, 

 in Korea, and in the north of Japan. These far eastern 

 Crayfishes, however, differ so much from the typical 

 species of Astacus that they are now placed in a 

 subgenus (sometimes regarded as a distinct genus), 

 Cambaroides. Curiously enough, the typical genus 

 Astacus reappears again on the other side of the 

 Pacific, where several species occur in that part of 

 North America which lies west of the Rocky Moun- 

 tains. East of the Rockies, again, numerous species 

 are found belonging to a distinct genus, Cambarus, 

 which ranges from Canada to Central America and 

 Cuba, and this genus is allied in certain respects to 

 the Cambaroides of Eastern Asia. If the systematic 

 relations of these genera have been properly inter- 

 preted, it is by no means easy to understand in what 

 way their present distribution has been brought 

 about. 



The Southern Crayfishes have an even more scat- 

 tered and discontinuous range. In New Zealand the 

 genus Paranephrops occurs, in Australia and Tasmania 

 the genera A stacopsis (Plate XX.), Cheraps&ndEngczus 

 (Plate XX.). A single species of Cheraps has been re- 

 12 



