COMMUNITIES IN STANDING WATERS 347 



only a single species in the other lakes, Tanganyika has 12, 11 belong- 

 ing to 3 genera of the Atyidae, Limnocaridina, Caridella, and AtyeUa, 

 all probably endemic. 57 Two of the fresh- water crabs belong to the 

 widespread African genus Potamon, 3 belong to the endemic genus 

 Platytelphiosa. 58 At least 9 gasteropod genera are endemic, and these 

 are notable for the resemblance of their shells to marine forms (Fig. 

 7) . These genera live mostly in deep water, the shallower levels being 

 populated by Planorbis, Limnaea, Ampullaria, Bithynia, and others. 

 The whole molluskan fauna has been reviewed by Pilsbry and Be- 

 quaert. 59 No less than 121 of 146 species of fishes are confined to this 

 lake; 89 are cichlids of which 84 species and 15 genera are endemic, 

 and there are 2 endemic genera of silurids. 60 



Lake Posso in Celebes agrees with Baikal and Tanganyika in 

 the individuality of its gasteropod fauna. There are 16 endemic forms 

 out of 21 species and subspecies. The general aspect of the fauna is 

 decidedly ancient; the melanias, with the exception of 2 widespread 

 forms, belong to the paleomelanias, distinguished by the structure 

 of their radula and operculum, and an endemic genus with swollen lip 

 is distinguished as Tylomelania. The two other lakes in the northern 

 part of the southwestern peninsula of Celebes, lying in the same tec- 

 tonic trough with Lake Posso, have a similarly peculiar fauna. ,n The 

 wealth of individuals is very large in Lake Posso, so great that the 

 dead shells in places thickly cover the shore. 



Lake Ochrida, between Jugoslavia and Albania, has been shown 

 to have a molluskan fauna rich in endemic forms, unlike those of 

 most European lakes, which include representatives of the otherwise 

 extinct Pliocene gasteropod fauna. 62 



The Caspian Sea recalls these lakes by similar deep-seated dif- 

 ferentiation of its fauna. The genus Pseudocuma has already been 

 discussed (p. 80). Nineteen out of 26 species of snails belong to en- 

 demic genera {Micromelania, Caspia, etc.). 63 The clupeid fishes have 

 10 endemic species of the genus Clupeonella in the Caspian. 64 Besides 

 these peculiar forms there are certainly relicts, such as the arctic 

 copepod Limnocalanus grimaldii which occurs also as a glacial relict 

 in the Gulfs of Finland and Bothnia. 65 Finally, there are also species 

 which have entered from fresh water. The Caspian does not seem 

 to have been isolated so completely or so long as Lakes Baikal and 

 Tanganyika or the lakes of Celebes. 



On account of their peculiar faunae, Baikal and Tanganyika have 

 been thought of as marine relict lakes. It is true that in Lake Baikal 

 certain forms such as the sponge Lubomirskia, which occurs in Ber- 

 ing Sea, and the polychaete Dybowscella of an otherwise marine 



