326 DISTRIBUTION AND ATIOLOGY OF THE CRAYFISHES. 
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 
Potamobiide obtained access to the river basins in which 
they are now found, 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 Atyide, and the fluviatile crabs (Thelphuside), 
being the only considerable groups among them which 
habitually confine themselves to fresh waters. But 
even in such a genus as Pengus, most of the species 
of which are exclusively marine, some, such as Peneus 
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, 
