300 DISTRIBUTION AND .ETIOLOGY OF THE CRAYFISHES. 



be remembered, are much less salt than ordinary sea 

 water. 



It will be observed that while the two forms, A. torren- 

 tium and A. nobilis, are intermixed over a large part of 

 Central Europe, A. torrentium has a wider north-west- 

 ward, south-westward, and south-eastward extension, 

 being the sole occupant of Britain, and apparently of 

 the greater part of Spain and of Greece. On the other 

 hand, in the northern and eastern parts of Central 

 Europe, A. nobilis appears to exist alone. 



Further to the east, a new form, Astacus leptodactylus 

 (fig. 75), makes its appearance. Whether A. leptodactylus 

 exists in the upper waters of the Danube, does not appear, 

 but in the lower Danube and in the Theiss it is the domi- 

 nant, if not the exclusive, crayfish. From hence it extends 

 through all the rivers which flow into the Black, Azov, 

 and Caspian Seas, from Bessarabia and Podolia on the 

 west, to the Ural mountains on the east. In fact, the 

 natural habitat of this crayfish appears to be the water- 

 shed of the Pontocaspian area, excluding that part of the 

 Black Sea which lies southward of the Caucasus on the 

 one hand, and of the mouths of the Danube on the other.* 



It is a remarkable circumstance that this crayfish not 

 only thrives in the brackish waters of the estuaries of 

 the rivers which debouche into the Black Sea and the 

 Sea of Azov, but that it is found even in the Salter 



* These statements rest on the authority of Kessler and Gerstfeldt, 

 in their memoirs already cited. 



