THE DISTRIBUTION OF CRABS AND CRAYFISHES. 337 
and fluviatile crabs (Thelphusa) compete for the posses- 
sion of the freshwaters; and it is not improbable that 
under some circumstances, they may be more than a 
match for crayfishes; so that the latter might either be 
driven out of territory they already occupied, as Astacus 
leptodactylus is driving out A. nobilis in the Russian 
rivers; or might be prevented from entering rivers already 
tenanted by their rivals. 
Tn connection with this speculation, it is worthy of 
remark that the area occupied by the fluviatile crabs is 
very nearly the same as that zone of the earth’s surface 
from which crayfish are excluded, or in which they are 
scanty. That is to say, they are found in the hotter 
parts of the eastern side of the two Americas, the West 
Indies, Africa, Madagascar, Southern Italy, Turkey and 
Greece, Hindostan, Burmah, China, Japan, and the 
Sandwich Islands. The large-clawed fluviatile prawns 
are found in the same regions of America, on both 
east and west coasts, in Africa, Southern Asia, the 
Moluceas, and the Philippine Islands; while the Atyide 
not only cover the same area, but reach Japan, extend 
over Polynesia, to the Sandwich Islands, on the north, 
and New Zealand, on the south, and are found on both 
shores of the Mediterranean; a blind form (Troglocaris 
Schmidtii), in the Adelsberg caves, representing the blind 
Cambarus of the caves of Kentucky. 
The hypothesis respecting the origin of crayfishes 
Zz 
