m 



HApri 



290 NORTHERN RUSSIA AND SIBERIA chap. 



the vast mountain ranges which terminate to the north the table 

 land of central Asia, at the eastern extremity of which it dips 

 sharply southwards, so as to include the Amoor basin and Corea. 



All the larger Helices are wanting, and no land operculates 

 occur. Helix arhustorwn L., S. nemoralis MiilL, II. lapicida L., 

 H. aculeata Miill., and Hyalinia nitidula Drap., do not appear to 

 occur east of the Baltic ; Arion fuseus Miill., Helix strigella Drap., 

 Buliminus obscurus Miill., Clausilia laminata Mont., C. hidentata 

 Bttg., C. plicatula Drap., Viviparus fasciatus Miill., and Neritina 

 fluviatilis L., do not pass the Urals. 



In the Obi district (West Siberia) a further batch of European 

 species find their easterly limit. Among these are Helix hispida 

 L., BitJiynia tentaculata L., Vivipara vivipara L., Pisidium amni- 

 cum MiilL, and Unio tumidus Retz. A few distinctly Siberian 

 species now appear, e.g. Ancylus sihiricus Gerst., Valvata sibirica 

 Midd., and Vitrina rugulosa Koch. 



The following are among the European species which reach 

 eastern Siberia : Hyalinia nitida Miill., Suecinea oblonga Drap., 

 Planorbis vortex L., spirorbis L., marginatus Drap., rotundatus 

 Poir., fontanus Light., Valvata piscinalis Miill., Bithynia ventri- 

 cosa Leach, and Anodonta variabilis Drap. Here first occur such 

 characteristic species as Physa sibirica West., P. aenigma West., 

 Helix pauper Gld., H Stuxbergi West., H Nordenskioldi West., 

 Planorbis borealis Lov., Valvata alienaWest., Cyclas nitida Cless., 

 and O. levinodis West. Li the Amoor district a decided Chinese 

 element makes its appearance in a few hardy forms which 

 have penetrated northward, e.g. Philomycus bilineatus Bens., 

 and a few each of the Fruticieola (Chinese) and Acusta groups 

 of Helix. Out of 53 species, however, enumerated from this 

 district, as many as 33, belonging to 18 genera, occur also in 

 Great Britain. 



Lake Baikal. — The MoUusca of Lake Baikal exhibit distinct 

 characteristics of their own, which seem to indicate the long- 

 continued existence of the lake in its present condition. 

 Several entirely peculiar genera occur, which are specialised 

 forms of Hydrobia^e.g. Baikalia, Liobaikalia^ G-erst/eldtia^ Dybow- 

 skia^ and Maackia; Benedictia alone extends to the basin of 

 the Amoor. Choanomphalus, another peculiar and ultra-dextral 

 (p. 250) genus belonging to the Limnaeidae, appears to be related 

 to the West American Oarinifex. 



