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



first of all in the questions touched on above concerning the role of the Ice 

 Lake Sea and the joining of the Yoldian Lake and White Sea. 



The so-called 'ribbon' clays are thought to be the characteristic type of 

 the deposits of the Ice Lake Sea. They have been discovered in a number of 

 places between the Baltic and White Seas at heights of up to 180 m (on the 

 shore of Lake Onega). The Baltic Ice Lake Sea covered a considerable area of 

 north and northwest Europe, leaving in the south large inlets which became 

 lakes. Some scientists connect with the waters of this Ice Lake Sea the appear- 

 ance of a number of cold-water relicts of marine origin (Hogbom-Thienemann 

 theory) in the lakes of northwest Europe of the Baltic Sea basin which lie 

 beyond the boundaries of subsequent phases of the Baltic Sea. When, where 

 and how the waters of this phase of the Baltic Sea came into contact with the 

 neighbouring seas has not yet been established, but the Baltic Ice Lake un- 

 doubtedly received a group of brackish-water forms from some neighbour- 

 ing semi-fresh or fresh body of water, distributed them after its regression 

 among individual remaining lakes of the Baltic basin, and transferred 

 them to the fauna of the Yoldian Sea. It may quite possibly have obtained its 

 relicts from the northeast; the well-known Mysis relicta, for instance, and 

 Pontoporeia affinis, Pallasea quadrispinosa, Limnocalanus grimaldi, Mesi- 

 dothea entomon, Myoxocephalus quadricornis and Osmerus eperlanus. This 

 theory of Hogbom-Thienemann is accepted by many scientists (Ekman and 

 others). E. Gams (1929) has spoken against this theory; he thinks that the 

 penetration of these organisms into the lakes of Denmark, northern Germany 

 and the northwestern part of the European u.s.s.r. should be connected with 

 the Yoldian transgression. 



The occurrence of relicts in the bodies of water outside the coastal bound- 

 aries of the Yoldian Sea is explained by Gams by passive transfer or active 

 migration and quick adaptation to fresh-water life, so that in these bodies 

 of water they are not relicts but immigrants according to Ekman's termino- 

 logy (relicts for the areas into which they were transferred or penetrated 

 at a second stage). Gams rejects any link between the history of these relicts 

 and the Ice Lake Sea. Many hydrobiologists, however, do not admit the 

 possibility of passive transfer or active migration of these animals from one 

 body of water to another, or against the current of a river. The migration 

 capabilities of such crayfish as Limnocalanus, Mysis, Pontoporeia, and of 

 Pallasea almost to the same extent, are very weak. Thus among the large 

 number of Scandinavian bodies of water investigated there is not a single one 

 situated above the mean boundaries of the Yoldian Sea which contains even 

 one of the four crustaceans mentioned. Their occurrence as a result of transfer 

 by birds or flying insects is, obviously, quite impossible. Therefore the passive 

 or active penetration of these crayfish into lakes which do not belong to the 

 Baltic Sea basin, as for example the Seliger Lake, is thus even more improbable. 



Hogbom's theory of ice lakes of large area and high level extending south 

 much farther than the boundaries of the Yoldian Sea, can be used to explain 

 problems of a biological nature which arise if the possibility of passive trans- 

 fer or active migration of relict crayfish from one body of water to another 

 is accepted. 



