342 



BIOLOGY OF THE SEAS OF THE U.S.S.R. 



The cold-water mollusc Yoldia (Portlandia) arctica is the most char- 

 acteristic form of the Yoldian Sea fauna known to us. Some other forms are, 

 however, equally characteristic, for example the diatom Campylodiscus 

 clypeus. Yoldia arctica, however, could have lived only in the most saline 

 western parts of the Sea. Myoxocephalus quadricomis and Phoca foetida in 

 the large Finnish lakes are probably remnants of this fauna, since these lakes 

 had been cut off from the Baltic Sea as early as the Littorina stage. The Yoldian 



Fig. 169. Conjectured role of Siberian Ice Lake at period of its greatest glaciation, 

 in evolution and distribution of glacial relicts. The lake is marked by cross-hatching : 

 1 and 2 : routes of exchange of glacial forms ; 3 and 5 : migration of forms of marine 

 origin ; 4 : migration of the ancestor of Pallasea quadrispinosa (possibly P. kessleri) 

 from Lake Baikal. Places of occurrence of P. quadrispinosa are marked by a circle : 

 a, in Nalim's Lake ; b, in river Lena estuary ; c, in Novaya Zemlya. Occurrence of 

 P. laevis (possibly descended from P. quadrispinosa) is indicated by a triangle. Occur- 

 rence of Mesidothea entomon in eastern Siberia is marked by crosses (Sagerstrale). 



Lake phase did not last long (barely 700 years according to Miinthe ; only for 

 500 years according to Sauramo, 1953). 



A rise of land in the western part of the Yoldian Lake cut it off from the 

 ocean ; it lost much of its salinity and turned into the low-salinity, closed 

 Ancylus Lake Sea with a powerful flow of water to the west into the North 

 Sea. This phase, according to De-Geer, lasted for 2,200 years. When this 

 body of water lost its salinity it became densely populated with fresh-water 

 forms, among them the molluscs Ancylus fiuviatilis and various species of 

 crustaceans and molluscs (Limnaea, Planorbis). 



Penetration of Atlantic fauna 



During the Littorina stage salinity off Gotland reached 12% (Fig. 136); now 

 this salinity is found only in the Belt. The Baltic Sea again became con- 

 nected with Lake Ladoga. For this phase the mollusc Tapes decussatus is 

 most characteristic. Mytilus edulis, Cardium edule, Hydrobia baltica, Neritella 



