300 DISTRIBUTION AND TIOLOGY 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 Caspiar 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 aremarkable 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, 
