THE CRAYFISH AND ITS ALLIES 99 



fishes of the region have probably succumbed. All the 

 islands now inhabited by crayfishes, such as England. 

 Japan, and Cuba, 1 were probably once connected with the 

 mainland. 



The northern subfamily of crayfishes contains, accord- 

 ing to Faxon, two genera Astacus and Cambarus of 

 which the latter can be subdivided into the subgenera 

 Cambarus and Cambaroides. These groups occupy dis- 

 tinct geographical areas. The genus Astacus is found, in 

 the Old World, in Europe and western Asia as far south 

 as the Aral and Caspian seas, and in America in the region 

 west of the Rocky Mountains, draining into the Great Salt 

 Lake and the Pacific Ocean. It is thus seen to occupy the 

 western sides of the two northern continents. Likewise 

 Cambarus and Cambaroides occupy the two eastern coasts 

 of the northern continents; for Cambarus is found in North 

 America east of the Rocky Mountains in the region 

 bounded on the north by Lake Winnipeg and New Bruns- 

 wick and on the south by Guatemala and Cuba, while 

 Cambaroides is limited to the Amoor River basin in Asia, 

 and to Japan. 



We thus find among the crayfishes what is known as 

 discontinuous genera; that is, genera which now occupy 

 widely separated areas, such as Astacus in Europe and 

 Pacific North America, but which once ranged over the 

 intervening regions as well. From some cause, the 

 struggle for existence became too severe in the intervening 

 regions, so that Astacus and Cambarus were annihilated on 

 the eastern and western sides of the continents respectively. 

 In southern Asia we find that the struggle was doubtless 

 with the successful river-crab. It is interesting to note 



1 It is doubtful whether Cuba has been connected with the mainland. 



