1902] AND ANCIENT GEOGRAPHY. 315 



cated. But before we do so, we have to say a few words by way 

 of explanation and introduction, characterizing the value of the 

 study of the freshwater Decapods for these purposes. 



In all the following discussions, the fundamental supposition has 

 been made that freshwater crayfishes, as well as freshwater crabs, 

 do not possess any exceptional means of dispersal ; that is to say, 

 that they are restricted to fresh water and cannot exist in salt water ; 

 that they cannot leave the water for any continued period, and con- 

 sequently cannot migrate over land to any extent ; and, finally, 

 that they do not possess in any stage of their life, and especially not 

 in the egg or larval stage, any means or devices which permit their 

 passive transport. We may specify these three points in the follow- 

 ing way : 



i. The restriction to fresh water is not absolute. There are a few 

 exceptions, namely : 



Potamobius pachy pus (Rthk.) is found in the Black and Caspian Seas 



in brackish and salt water. 

 Potamobius trowbridgei (Stps.) has once been collected in salt water 



at Monterey, California (Faxon, 1898, p. 666). 

 Cambarus uhleri Fax. is characteristic for the marshes of the coast 



of Maryland, and lives in fresh, brackish, and salt water. 

 Cambarus montezuma Sauss. has been found, in one case, in a salt 



lake in Mexico (Lake Tezcoco, near City of Mexico ; see 



Faxon, 1885, P- [2 3)- 

 Potamon fluviatile var. ibericum (Bisb.) is found in fresh water and 



salt water of the Caspian Sea (see Ortmann, 1897, P- 3° 2 )- 



On account of the small number of these cases, we have to re- 

 gard them as exceptional, and they are, no doubt, secondary 

 adaptations. In fact, none of these species is a true saltwater 

 form, they being always more or less euryhalin, and frequenting 

 also brackish or freshwater. Thus we may say, generally, that 

 both, crayfishes and crabs, discussed here, are true freshwater ani- 

 mals, and preeminently so, and that a migration across oceans or 

 parts of oceans is practically excluded. 



2. Being animals breathing by gills, crayfishes and freshwater 

 crabs cannot leave the water. This rule is without exception with 

 the Potamobiidce and Parastacidce ; they may leave the water for a 

 short time, but a prolonged stay outside of it is always fatal. There 

 are only a few species in North and South America, and in Aus- 



