290 OKTMANN — DISTRIBUTION OF DECAPODS [April 3 



Pacific Ocean and finally immigrated into Northwestern America. 

 A trace of the direction of this route is preserved in the presence of 

 Potamobius nigrescens near Unalaska. After the final geographical 

 separation of the European and American descendants from the 

 original group in Eastern Asia each of the three groups developed 

 independently, and the Asiatic group acquired several more advanced 

 characters (copulatory organs and hooks) which otherwise are 

 found only in Cambarus, but which do not point to a closer affinity 

 to the latter genus, but are only due to parallelism. 



Further, the West American Potamobii possess a character that is 

 found also in Cambarus. Faxon mentions that the second pleopods 

 of the male resemble not only those of Cambaroides, but also those 

 of Cambarus, while the European species are different in this 

 respect. This would bring the genus Cambarus into closer relation 

 to the West American Potamobii, and although this similarity 

 would hardly be of much value by itself, we have to regard it as 

 significant, since it agrees well with the distributional facts. The 

 tracing back of Cambarus to Cambaroides is geographically impos- 

 sible, and just this latter difficulty has induced the writer to exam- 

 ine more closely the supposed resemblance of both, and the result 

 is as has been discussed above. A closer connection of the Euro- 

 pean species of Potamobius with Cambarus is out of the question, 1 

 and thus only the third group is left, the West American Potamobii. 



From the latter group Cambarus is very sharply distinguished 

 though and no transitional forms are known. Probably this is due 

 to the fact that the connection of the area of both is far remote 

 geologically — that is to say, that the migration of Potamobius into 

 Mexico is very old and that the separation of both genera took 

 place in very early times, the one becoming restricted to North- 

 western America (southward to California), the other developing 

 on the Mexican plateau out of the old Potamobius stock that origi- 

 nally immigrated thither from the North. Thus the differential 

 characters of Cambarus became well fixed and no transitions to the 

 old stock are found any more. 



Thus for the family of the Potamobiidcz we may express the fol- 



1 Faxon (1885, p. 176) thinks that in former times Cambarus and Potamobius 

 occupied about the same area, and in order to support this he mentions the sup- 

 posed existence of a blind Cambarus in the caves of Carniola, Austria. How- 

 ever, this latter record is entirely erroneous. There exists no Cambarus in the 

 caves of Carniola (see Haman, Europaische Hoehlenfauna, 1896). 



