1902.] AND ANCIENT GEOGRAPHY. 289 



Stan, and northward the area reaches the White Sea. East of the 

 Ural Mountains the genus is said to be lacking, but it is found (the 

 widely-distributed species P. leptodaciylus) introduced in the river 

 Obi and its affluents. Some observations, however, have been 

 made which render it possible that F. leptodaciylus is an original 

 inhabitant of these parts. 



As Huxley (1879) and Faxon (1885, p. 140) believe, thedifferent 

 forms of Potamobiiis have immigrated into Europe from the East, 

 and we can distinguish an older immigration on the part of the 

 group formed by the species P. pallipes and toi-reniium and a more 

 recent one on the part of P. astaciis and its allies. And even 

 within the latter group it seems that P. astaciis is older than the 

 other species and that it is pushed gradually westward by P. lepto- 

 daciylus, which is spreading in a westerly direction. The writer is 

 of the same opinion, and we shall see below that this is the only 

 theory that is admissible, if we consider the origin of Europe as a 

 continental mass. The occupation of Europe, after it had lost the 

 character of an archipelago and become part of the Eurasiatic con- 

 tinent, was possible for these animals only in a west-easterly direc- 

 tion. This corresponds also to the fact that those forms allied to 

 the European Potamobii, which are the nearest geographically, are 

 found to the east of them. They are the forms of Ca77ibaroides in 

 Eastern Asia, and we can readily imagine that from the area of dis- 

 tribution of Cambaroides an extension existed formerly in a westerly 

 direction across Central Asia, which connected with the European 

 area of Pota7?iobius, and this connection represents the direction of 

 the migration. 



The forms of Potamobiiis which are found in Western North 

 America possess a continuous area of distribution ^ which is separated 

 from the rest of the genus. Huxley and Faxon, as has been men- 

 tioned above, believe that these American species are more closely 

 related to the European, but I think we have reason to accept a 

 different view. 



My opinion is that a primitive group, which was ancestral to all 

 three of the living groups, formerly existed in Eastern Asia, which 

 is to be regarded as the centre of origin of the Potamobudce. This 

 group sent out a branch in a westerly direction, which finally 

 reached Europe, and it also sent out a branch in an easterly direc- 

 tion, which migrated apparently along the northern shores of the 



1 Possibly with the exception of the isolated station near Unalaska. 



