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 opei'culates 

 occur. Helix arbustorum L., II. nemoralis MiilL, JI. lapieida L., 

 H, aculeata MiilL, and Hyalmia nitidida Drap., do not appear to 

 occur east of the Baltic ; Avion fuscus MiilL, Helix strigella Drap., 

 Buliminus ohscurus MiilL, Olausilia laminata Mont., O. hidentata 

 Bttg., C. pUcatula Drap., Viviparus fasciatus MiilL, and Neritina 

 fiuviatilis 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., Bithyiiia tentaculata L., Vivipara vivipara L., Pisidiuin ainni- 

 eum 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, Succinea oblonga Drap., 

 Planorbls vortex L., spirorbis L., margiiiatus Drap., rotundatus 

 Poir., fo7itaniis Light., Valvata piscinalis MiilL, Bitliynia ventri- 

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

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

 Helix pauper GkL, H Stuxbergi West., H JVordenskioldi West., 

 Planorbis borealis Lov., Valvata alienaWest.^ Cyclas nitida 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. Pliilomycus bilineatus 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 Mollusca 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^ Grerstfeldtia^ Dyboiv- 

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

 the Amoor. Choanoynphalus^ another peculiar and ultra-dextral 

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

 to the West American Carinifex. 



