1902.] AND ANCIENT GEOGRAPHY. 377 



the beginning of the Pleistocene, was covered by the Sarmatian and 

 Ponto-Caspian Sea. This group consequently can only have 

 reached these parts at a later period, namely, in Interglacial or 

 Postglacial times, and its immigration no doubt corresponds to the 

 Siberian of Scharff (/. c, pp. 448 and 466). In regard to the other 

 group formed by the two species, P. pallipes and to?'rentium, which 

 occupy the South and West of Europe, we have to call attention to 

 the important fact that this group is found not only in Southern and 

 Western Europe, but also in England. Now we know that England 

 was connected with the continent in Preglacial and even during 

 Glacial times, and that this connection existed up to the beginning 

 of the Siberian migration (Interglacial). It was interrupted later — 

 according to Osborn (1900, p. 572), at about this time (Middle 

 Pleistocene) and possibly even later. ^ 



The fact that Potamobius astacus is found in France, but not in 

 England, while P. pallipes passes over into the latter country, 

 points to a difference in time of the immigration of either species. 

 P. pallipes arrived in these parts before the end of the Glacial time, 

 P. astacus at the end of it or even later. The latter consequently 

 without doubt belongs to the Siberian migration, but rather to the 

 later part of it. P. pallipes may belong to the earlier Interglacial 

 part of the Siberian migration and have come from East and Cen- 

 tral Europe; but it is also possible that it belongs to Scharff's 

 southern migration and came from Asia Minor over the Balkan 

 Peninsula. It is true, forms of the pallipes group have not been 

 found in Asia Minor nor in Algiers, but it is not impossible that 

 such may be discovered in these parts, or that they once existed 

 there and have now disappeared. We shall see below why this 

 latter assumption is admissible. The crayfishes in Asia Minor, 

 Southern Italy and iVlgiers may have been exterminated by the 

 freshwater crabs subsequently occupying these parts. Until this 

 question is finally settled it is impossible to decide whether the 

 gxoxr^oi P. pallipes has reached its present area by the southern 

 route (Miocene-Pliocene) or by the route of the Siberian migration 

 (end of the Pleistocene) ; but however that may be it arrived in 

 Europe before the group of P. astacus. 



The connection of the European crayfishes with the Sinic conti- 



^ Suess (1888, p. 528) thinks it possible that this happened in historic time or 

 shortly before the beginning of it. 



