. Biology of the seas of the Marine biology -- Soviet Union; Hydrology -- Soviet Union. 342 BIOLOGY OF THE SEAS OF THE 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
. Biology of the seas of the Marine biology -- Soviet Union; Hydrology -- Soviet Union. 342 BIOLOGY OF THE SEAS OF THE 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 Litt
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