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 arhustorum L., If. nemoralis Miill., U. lapicida L., 

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

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

 Bidiminus ohscurus Miill, Glausilia Imninata Mont., G. hidentala 

 Bttg., C. plicatula Drap., Viviparus fasciatns Miill., and NerUina 

 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., Bithj/nia tentaculata L., Vivipara vivipara L., Pisidium amni- 

 cum Miill., and ITnio tumidus Eetz. A few distinctly Siberian 

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

 Midd., and Vitrina rugtdosa Koch. 



The following are among the European species which reach 

 eastern Siberia : Hycdinia nitida Miill., Succinea oUonga Drap., 

 Planorhis vortex L., spirorhis L., marginatus Drap., rotundatus 

 'PoiT.,fontanus JAght., Valvata piscincdis Miill., Bithynia ventri- 

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

 characteristic species as Fhysa sihirica West., P. aenigma West., 

 Helix pauper Grid., H. Stuxlergi West., H Nordenskioldi West., 

 Planorhis horealis Lov., Valvata aliena West., Cyclas 7iitida Cless., 

 and C. levinodis West. In the Amoor district a decided 

 Chinese element makes its appearance in a few hardy forms 

 which have penetrated northward, e.g. Philomycus hilineatus 

 Bens., and a few each of the Fruticicola (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 Hydrohia, e.g. Baikalia, Liohaikalia, Gerstfeldtia, Dyhow- 

 skia, and Maaekia ; Benedictia alone extends to the basin of the 

 Amoor. Ghoanomphalus, another peculiar and ultra-dextral (p. 

 250) genus belonging to the Limnaeidae, appears to be related to 

 the West American Garinifex. 



