316 OKTMA^'N — DISTRIBUTION OF DECAPODS [Aprils, 



tralia, burrowing in mud, which leave the water habitually; but 

 they always have to return to the water to moisten their gills, and 

 their burrows end in water. The forms most adapted to a subter- 

 restrial life are probably the two species of Engceiis in Tasmania. In 

 general, yi?r the crayjishes, tracts of land without water {^deserts) are 

 absolute barriers. 



The PotamobiidcE lead a rather amphibic life and leave the water, 

 in many cases, habitually. Yet they always depend on the presence 

 of water and cannot go far out of easy reach of it. Some of the 

 species (^Potamon fluviatile in Persia, etc.) live in steppes, where 

 there is a scarcity of water, but here they always are found near 

 some kind of water supply. In general, they also cannot exist in 

 deserts. 



3. As in all other Decapods, also in crayfishes and freshwater 

 crabs the eggs are carried and hatched under the abdomen of the 

 female. There is, as far as we know, no free metamorphosis of the 

 young (known in PotamobiuSj Camba^^tis, Potamon), and the young 

 hatch in a stage similar to the parents. Thus there seems to be no 

 means which effect, under normal conditions, an increased facility 

 of dispersal in an active or passive way among the young ones. 

 There may be, occasionally, a passive transport by other animals 

 (water fowl), but such cases can only be exceptions and have never 

 been observed. The whole character of the distribution of the 

 different species is against the assumption of exceptional means of 

 dispersal. 



I. CONNECTION OF NORTHEAST ASIA WITH NORTHWEST AMERICA BY 

 WAY OF BERING SEA. 



A connection of northeast Asia with northwest America is pos- 

 tulated, as we have seen above, by the presence of Potamobiidoi in 

 the region of the Amur river, Korea, and north Japan on the 

 one side, and in western North America on the other ; the direc- 

 tion of this connection is indicated by the presence of Poiamobius 

 nigrescens (Stps.) in Unalaska. 



This connection is mentioned by Jacobi (1900) under his 

 "regions of dispersal " (" Ausbreitungsgebiete "), and is called 

 by the name of *' Berings-Strassen-Ausbreitungsgebiet." This is 

 well known among zoogeographers. In fact, for an explanation of 

 the very peculiar conditions of distribution of many animals of the 

 northern hemisphere, a former connection of the northern land 



