302 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 Bion, 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 Flusskrebae, 1. c. p. 369-70), has an in- 

 teresting discussion of this question. 



