348 ANIMALS IN INLAND WATERS 



group, suggest a marine relation. In Lake Tanganyika, the fresh- 

 water medusa Limnocnida tanganyicae, the copepod Schizopera, and 

 the snails with a marine habitus (referred to as "halolimnic" by 

 Moore), 66 are the basis for the theory of marine connection. These are 

 scarcely more than exceptions, however, and in view of the fact that- 

 all fresh-water animals which are primarily aquatic are ultimately of 

 marine derivation, the marine habitus of the rare and especially the 

 ancient forms, contrasting with the more recent and widespread ones, 

 cannot be taken as direct or conclusive evidence of marine derivation. 

 The faunae of these lakes simply represent the only existing, ancient, 

 fresh-water faunae. The snails of Tanganyika, supposed by Moore to 

 be marine relicts, are in part strikingly similar to fresh-water fossils, 

 Paramelania (Fig. 76), to Pyrgulifera, Syrnolepsis with Fascinella 

 from the Lower and Middle Cretaceous of Europe and America. 67 

 No snails similar to those of Tanganyika are found in the other lakes 

 of Africa, and in relict lakes whose origin is not a matter of specula- 

 tion, the related faunae are found in near-by waters. In the lakes of 

 Sweden and Latvia, for example, Cottus quadricornis, Mysis relicta, 

 and Pontoporeia affinis have Baltic relatives. It seems that the more 

 recent forms have crowded the ancient ones in part into the greater 

 depths in Tanganyika; Tiphobia (Fig. 7a) and Bathyanalia are found 

 at depths of 75 and 110 m. 



The idea that Lake Baikal has never been marine is supported by 

 the fact that all the geologic formations of the surrounding region 

 are of fresh-water or terrestrial origin. 68 A summary of this problem 

 for the Baikal fauna is presented by Berg, 68 supporting the theory 

 or non-marine origin; for the contrary theory the reader may be re- 

 ferred to Wereschtschagin, 52 and Cockerell. 50 



Small bodies of water. — Small bodies of water are distinguished 

 from lakes by the absence of a permanent and continuous open water 

 surface in the center; in spring parts of these waters may be free from 

 plant growth, but with increasing warmth this open water becomes 

 more and more reduced until only small spots remain or it is entirely 

 gone. The littoral flora extends into the deepest parts of such waters. 

 They often represent stages of old age of former eutrophic lakes. De- 

 posits of muck fill up the lake; the plant communities of the shores 

 penetrate farther and farther; Chara overruns the bottom, and mol- 

 lusks and other animals follow ; the open water of the surface becomes 

 more and more restricted by Potamogeton, water lilies (Nymphaea) , 

 and other plants. Thus ponds, pools, and marshes arise, and by further 

 development, low and finally high prairie. The conditions of oxygen 

 supply and temperature also change together with these developments. 



