THE HAUNTS OF LIFE 113 



strong evidence to show that the fauna of Lake Tanganyika 

 includes many molluscs, for instance, which were inhabi- 

 tants of Jurassic seas. It is very striking to find in Lake 

 Tanganyika a Gasteropod like Typhobia horei — whose 

 kinship is certaialy with marine types. 



Several different kinds of freshwater Medusoids (Limno- 

 codium, Limnocnida) are known from various parts of 

 the world, and are probably to be interpreted as rehcts of 

 a mariae stock. The same may be said of the very simple 

 freshwater polj^, Microhydra ryderi, reported from North 

 America and also from Germany. Like numerous marine 

 hydroids, but unHke the common freshwater Hydra, it 

 liberates a minute swimming-beU or Medusoid. 



It is necessary to distinguish between reUct marine 

 faunas and reMct seas. Thus the remarkable population 

 of Lake Baikal seems to be in part a rehct marine fauna, 

 but there is no evidence in the surrounding deposits to 

 show that the Lake was ever anything but a freshwater 

 basin. We must therefore suppose that the marine types 

 in the lake — the seals, for instance — migijated from an 

 ancient sea, along paths now hidden. ^ 



Thirty-four fishes are known from Lake Baikal, and 

 L. S. Berg divides them into those which are general in 

 Siberian freshwaters (17) and those (also 17) which are 

 endemic. Of the latter some are related to Siberian forms. 

 While others (Abyssocotini, Cottocomephoridse and Come- 

 phoridse) seem quite imique. There are no forms in the 

 Siberian waters, nor in the Arctic Ocean, nor in the Pacific 

 which come near these puzzling forms which Berg regards 

 as very ancient, and perhaps native (autochthonous) to 

 the Lake. They hve at greater depths than any other 

 freshwater fishes, descending to 1,600 metres. 



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