330 DISTRIBUTION AND ZTIOLOGY OF THE CRAYFISHES, 
brackish water lagoons of the Gulf of Mexico, but I 
am not aware that any of them have yet been met with 
in the sea itself. The Palemon lacustris (Anchistia 
migratoria, Heller) abounds in fresh-water ditches and 
canals between Padua and Venice, and in the Lago di 
Garda, as well as in the brooks of Dalmatia; but its 
occurrence in the Adriatic or the Mediterranean, which 
has been asserted, appears to be doubtful. So the Nile 
prawn, though very similar to some Mediterranean 
prawns, does not seem to be identical with any at 
present known.* ° 
Tn all these cases, it appears reasonable to apply the 
analogy of the Mysis relicta, and to suppose that the 
fluviatile prawns are simply the result of the adaptive 
modification of species which, like their congeners, were 
primitively marine. 
But if the existing sea prawns were to die out, or to 
be beaten in the struggle for existence, we should have, 
scattered over the world in isolated river basins, more 
or less distinct species of freshwater prawns,t the areas 
inhabited by which might hereafter be indefinitely en- 
larged or diminished, by alteration in the elevation of the 
* Heller, “Die Crustaceen des siidlichen Europas,” p. 259. Klunzinger, 
“ Ueber eine Siisswasser-crustacee im Nil,” with the notes by von Mar- 
tens and von Siebold: Zeitschrift fir Wissenschaftliche Zoologie, 1866. 
+ This seems actually to have happened in the case of the widely- 
spread allies and companions of the fluviatile prawns, Atya and Cari- 
dina. 1am not aware that truly marine species of these genera are 
known. 
