302 DISTEIBUTION AND .ETIOLOGY OF THE CRAYFISHES. 



southern parts of tbe Casi^ian, in which it lives at 

 considerable depths. 



In the north, Astactts Icptodactylus 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. nohilis 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 

 diff'erent crayfish, which has been called Astacus pachypus, 

 occurs; another closely allied form {A. angidosus) is met 

 with in the mountain streams of the Crimea and of the 

 northern face of the Caucasus ; and a third, A. colcliicus, 

 has recently been discovered in the Eion, or Phasis of 

 the ancients, which flows into the eastern extremitj' of 

 the Black Sea. 



With respect to the question whether these Ponto- 

 caspian crayfishes are si)ecifically distinct from one 

 another, and whether the most widely distributed kind, 

 A. leptodactylus, is distinct from A. nohilis, exactly the 

 same difficulties arise as in the case of the west European 



* Kessler (Die Russischen Flusskrebse, 1. c. p. 3G0-70), has an in- 

 teresting discussion of this question. 



