826 DISTRIBUTION AND AETIOLOGY OF TPIE CEAYFISHES. 



Danube into connection, first with the Mediterranean and 

 then with the western Atlantic ; and, as in former times, 

 it gave access from the south to the vast area now 

 drained by the Volga. When the Black Sea communi- 

 cated with the Aralo- Caspian sea, and this opened to 

 the north into the Arctic sea, a chain of great inland 

 waters must have skirted the eastern frontier of Europe, 

 just such as would now lie on the eastern frontier of Asia 

 if the present coast underwent elevation. 



Supposing, however, that the ancestral forms of the 

 Potamohiidce obtained access to the river basins in which 

 they are now foimd, from the north, the hypothesis that 

 a mass of fresh water once occupied a great part of the 

 region which is now Siberia and the Arctic Ocean, would 

 be hardly tenable, and it is, in fact, wholly unnecessary 

 for our present purpose. 



The vast majority of the stalk- eyed crustaceans are, and 

 always have been, exclusively marine animals ; the cray- 

 fishes, the Atyidce, and the fluviatile crabs {Thelphusidce), 

 being the only considerable groups among them which 

 habitually confine themselves to fresh waters. But 

 even in such a genus as Penceus, most of the species 

 of which are exclusively marine, some, such as Pencsus 

 brasiliensis, ascend rivers for long distances. More- 

 over, there are cases in which it cannot be doubted 

 that the descendants of marine Crustacea have gradually 

 accustomed themselves to fresh water conditions, and 

 have, at the same time, become more or less modified, 



