218 



their dispersal has 'been caused by any exceptional or abnormal 

 means of transport. Few zoologists will consequently dis- 

 agree with Dr. Ortmann's * assertion that fresh-water cray- 

 fishes are among the most important animals in so far as the 

 study of their distribution elucidates past changes of land 

 and water over the globe. The crayfishes have been brought 

 into great prominence by Professor Huxley's well-known 

 treatise on the subject. More recently it is principally in 

 America that their structure and distribution have been 

 studied with great assiduity. Dr. Faxon f was the first to 

 recognise that besides the American genera Potamobius 

 (Astacus) and Cambarus, there is a third genus of fresh- 

 water crayfishes which inhabits north-eastern Asia. The 

 latter, it is true, is only considered a sub-genus of Pota- 

 mobius by Dr. Faxon and also by Dr. Ortmann, but, as 

 Mr. Stebbing $ has pointed out, its intermediate position 

 between Potamobius and Cambarus entitles it to rank as the 

 distinct genus Cambaroides. 



The geographical distribution of these crayfishes (Pota- 

 mobiidae) is very suggestive and interesting. Europe is the 

 headquarters of the old and well-known genus Astacus, which 

 name, in the unfortunate search for priority, has had to 

 give way to Potamobius. The genus ranges practically 

 throughout Europe, from north to south and from east to 

 west, and only very little beyond it. Beyond the Caucasus 

 it crosses into Transcaucasia, Turkestan and western Siberia. 

 It is quite unknown in the remainder of Asia. In the rivers 

 of eastern Asia, in Korea, Japan and eastern Siberia we meet 

 only with members of the small group Cambaroides referred 

 to. The somewhat close resemblance of this Asiatic genus 

 to the American Cambarus does not point to blood relation- 

 ship, according to Dr. Ortmann, merely to convergence. In 

 America we find not only Cambarus, but also Potamobius, the 

 European crayfish, the latter genus being in America entirely 

 confined to the western States. Professor Huxley and Dr. 

 Faxon both urged that the American species of Potamobius re- 

 sembled the European crayfishes much more than the Asiatic 



* Ortmann, A. E., " Distribution of Freshwater Decapods," pp. 315 316. 

 t Faxon, W., "Kevision of Astacidae." 

 J Stebbing, T. E. E., "Crustacea," p. 208. 



