3802 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES. 
southern parts of the Caspian, in which it lives at 
considerable depths. 
In the north, Astacus leptodactylus is met with in the 
rivers which flow into the White Sea, as well as in many 
streams and lakes about the Gulf of Finland. But it 
has probably been introduced into these streams by the 
canals which have been constructed to connect the basin 
of the Volga with the rivers which flow into the Baltic 
and into the White Sea. In the latter, the invading A. 
leptodactylus is everywhere overcoming and driving out 
A. nobilis in the struggle for existence, apparently in 
virtue of its more rapid multiplication.* 
In the Caspian and in the brackish waters of the 
estuaries of the Dniester and the Bug, a somewhat 
different crayfish, which has been called Astacus pachypus, 
occurs; another closely allied form (A. angulosus) is met 
with in the mountain streams of the Crimea and of the 
northern face of the Caucasus; and a third, A. colchicus, 
has recently been discovered in the Rion, or Phasis of 
the ancients, which flows into the eastern extremity of 
the Black Sea. 
With respect to the question whether these Ponto- 
caspian crayfishes are specifically distinct from one 
another, and whether the most widely distributed kind, 
A. leptodactylus, is distinct from A. nobilis, exactly the 
same difficulties arise as in the case of the west European 
* Kessler (Die Russischen Flusskrebse, 1.¢. p. 369-70), has an ine 
teresting discussion of this question. 
