MOLLUSK-FAUNA OF ALASKA AND SIBERIA. 365 



modify essentially the general rule, and like the valleys themselves, 

 these species disappear a short distance north of the 49th parallel. I 

 was able to show, some thirty-five years ago, that in a broad way the 

 distribution of the birds and plants presents analogous features. 



Fauna of Northeastern Asia. 



The land shell fauna of the northeast extremity of Asia has little 

 individuality, but represents a mingling of the depauperated extremes 

 of the faunas of northeast China and of Europe with that series of 

 species which is sometimes called the circumpolar or circumboreal 

 fauna. 



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

 sufficient collecting, but even when the most generous allowance for 

 this factor is made, it still remains certain that the molluscan popula- 

 tion is far less in variety than might reasonably be expected. 



The palaearctic 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 ap- 

 preciable extent. It is possible that the comparatively recent emerg- 

 ence 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 pecu- 

 liarities of the Baikal faunula. 



Southeast of the Stanovoi range we find between the mountains 

 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 intimately 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 a few species which seem to be common to the shores 

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

 Pyramidula pauper, Punctum conspectum, and Anodonta beringiana. 



