1902.] AND ANCIENT GEOGRAPHY. 287 



Generally speaking the range of the genus Potamobius exhibits a 

 striking discontinuity, which has often been discussed. One group 

 of species occupies a continuous area in Europe (and Western 

 Asia); another in East Asia ; a third in Western North America} 

 It has been said that it is another remarkable fact that the American 

 species resemble the European more than they do the East Asiatic, 

 and that the latter more approach Cambarus, which idea is ex- 

 pressed by their position in a separate subgenus named Camba- 

 roides. But as regards the gills and the general form of the body, 2 

 Cambaroides belongs without question to Potamobius. The male 

 copulating organs are as different from those of Cambarus as they 

 are from those of the typical species of Potamobius, and the only 

 character that points decidedly to Cambarus is the presence of 

 copulatory hooks on the ischiopodites of certain peraeopods. But 

 also in this respect Cambaroides is rather peculiar, since these hooks 

 are found on the second and third pair, which case is represented 

 among Cambarus only in the fifth group (containing only three 

 species), while all the rest of the numerous species of this genus 

 possess these hooks either on the third and fourth or only the third 

 pair. 



I am of the opinion that the resemblance of Cambaroides to 

 Cambarus does not express very close blood relationship, but is due 

 to convergency. The development of hooks on the peraeopods of 

 the male, which serve, as is now known, the purpose of taking hold 

 of the female in copulation, is easily understood, if we remember 

 the manner in which copulation is performed, and it is also easily 

 intelligible that this device has possibly developed independently 

 in Cambaroides and Cambarus. The shape of the copulating 

 organs, which shows no doubt in Cambaroides a certain similarity 

 to the Cambarus type, can be explained in the same way, since it 

 is quite clear that if they are used in the same manner they may 



1 To the latter area belongs an isolated locality of P. nigrescens in Alaska. 

 According to Hay (1899) this species is found all along the western coast of 

 North America, from California to Alaska. To my knowledge intermediate locali- 

 ties between Washington and Alaska have not been published. 



2 Faxon (1885, p. 126) calls the shape of the body "subcylindrical," and says 

 that it resembles that of Cambarus. I cannot concur with him in this opinion; 

 the form of the carapace in Cambaroides is decidedly rather oval, as in Potamo- 

 bius, and besides there are variations also in this respect within the genus Cam- 

 barus. 



