92 THE WANDERINGS OF ANIMALS [CH. 



(2) the Deckeniae in East Africa and one species on 

 the Seychelles ; (3) the Potamocarcinae in tropical 

 America, including Antilles. Thus they occupy the 

 whole intertropical belt and this divides the cray- 

 fishes into the northern Potamobiidae and the 

 southern Parastacidae. Only in a few countries 

 do crabs and crayfishes slightly overlap, e.g. South 

 Mexico and Papua. Crabs are destructive to the 

 crayfishes, so that the latter cannot enter countries 

 already tenanted by the former. Crayfishes are 

 assumed to have originated during the Cretaceous 

 in Southern Asia, where they no longer exist. 



Their southern descendants, the Parastacidae, 

 survive in New Zealand, Fiji, Melanesia, New Guinea, 

 Australia and Tasmania, whence they crossed by 

 Antarctica into the southern temperate part of South 

 America. There is also one solitary form, Astacoides 

 madagascariensis, in Madagascar. 



The northern descendants of the ancient cray- 

 fishes are the Potamobiidae, comprising only three 

 genera. Astacus (e.g. the common crayfish, renamed 

 Potamobius by purists) in Europe and Western 

 Siberia, and in the North-West United States. The 

 discontinuous range is mitigated by a few species in 

 the Amur, Korea and North Japan, where they form 

 the sub-genus Cambaroides. Lastly Cambarus, to 

 which Astacus is directly ancestral, in the eastern 

 half of North America, Mexico and Cuba. 



