12 LAND AND FRESH WATER MOLLUSKS 



Aplexa hypnorum is known from northern Europe, western 

 Siberia, and the Chukchi Peninsula, but has not been reported 

 from eastern Siberia, or the Amur, though abundant in Alaska, 

 and reaching on the Taimyr Peninsula to 73° 30' north latitude. 



Zoogenites harfa is known from northern Scandinavia in 

 Europe ; from northeastern America, the Hudson Bay territory 

 and Southeastern Alaska, in America ; but in Siberia it is re- 

 corded only from the easternmost margin, the Chukchi Penin- 

 sula, Bering Island, Kamchatka and the lower Amur. These 

 singularities of distribution must await much more extended 

 knowledge before they can be adequately discussed, but it is 

 believed that to some extent they are due to the transgression 

 of the sea, or of glacial ice, over part of the area in which a 

 species might naturally be expected to occur, thus delaying the 

 occupation of the entire region by the species concerned. 



In the following table the distribution is indicated by the 

 headings of the six columns. Varieties are not included when 

 the typical form appears in the table. 



Column ' Eur.' includes those forms recorded as found in Eu- 

 rope, including the whole of European Russia and the Caucasus. 



Column ' Lena ' includes the drainage of the Lena and the 

 whole of Siberia from the Lena westward to the Ural Moun- 

 tains. It should be noted that a number of species which reach 

 the Lena from the west do not cross the Stanovoi Range. 



Column 'Amur ' includes the Amur drainage basin, the Island 

 of Sakhalin, and the smaller drainage areas between the Amur 

 and the Stanovoi Range. 



Column 'China' includes those forms which, having their 

 center of distribution in China or Japan, extend their range to 

 the drainage basin of the Amur, though often reaching only the 

 southern and eastern part of it. 



Column ' Kam.' indicates species belonging to the area in- 

 cluded in the Kuril Islands, the Commander Islands, Kamchatka 

 proper, the Chukchi Peninsula, and northeastern Siberia east of 

 the Stanovoi Range and north of Aian. 



Column 'Am.' includes those forms found in the Aleutian 

 Islands, northern and northwest America, which also occur on 

 the Asiatic side. 



