lO LAND AND FRESH WATER MOLLUSKS 



Much of the apparent poverty of the fauna may be due to 

 insufficient collecting, but even when the most generous allow- 

 ance for this factor is made, it still remains certain that the 

 molluscan population is far less in variety than might reasonably 

 be expected. 



The Palearctic fauna of Europe appears to extend clear 

 across northern Asia, losing a large proportion of its species on 

 the way, until (if the circumboreal species be excluded) only 

 about thirty species reach the headwaters of the Lena and the 

 barrier of the Stanovoi Range. A very remarkable local fauna 

 exists in the great ' relicten-see ' of Siberia, Lake Baikal, but 

 it does not appear to have tinctured the east Siberian fresh 

 water fauna outside of that lake, to any appreciable extent. 

 It is possible that the comparatively recent emergence of a large 

 part of eastern Siberia from the sea, and the presence of the 

 vast desert region to the south and west, may enter into the 

 explanation of this sparse shell fauna, as well as of some of the 

 peculiarities of the Baikal faunula. 



Southeast of the Stanovoi Range we find between the moun- 

 tains and the sea, the valley of the Amur and several smaller 

 valleys, such as the drainage basins of the Ud and the Tugar. 

 To the southwest the sources of the Amur emerge from the 

 deserts of Gobi and Dauria, and along the line of these water 

 courses has crept a certain number of molluscan forms inti- 

 mately related to or identical with those of Mongolia, China, 

 and the Orient. This forms the second element of the fauna 

 of northeast Siberia. The number of purely endemic species 

 is remarkably small, and a portion of those claimed to be of 

 this character are probably mere local mutations of widespread 

 Palearctic forms already known. Yet it would seem as if a 

 more thorough exploration must add largely to the species 

 now known, and it is almost incredible that the luxuriant 

 fertile valleys of Kamchatka and the innumerable streams 

 and lakes of that country should not be well populated with 

 mollusks. 



There are few species which seem to be common to the shores 

 of Bering Sea, both Asiatic and American, such as Succinea 

 chrysis^ Punctum cons-pectum and Anodonta heringiana. There 



