610 



DR. W. A. CUNNINGTON ON TflE 



of magnesium, and that of Kivu contains excessive quantities of 

 the latter as well as sodium salts. It is likely that the salinity 

 of Tanganyika was greater formerly, and may still be subject to 

 variation. Evidence exists of a considerable rise and fall in the 

 level of the lake, yet it seems probable that rainfall and evapora- 

 tion are very nearly balanced. There is reason to believe that 

 Tanganyi!<:a had no outlet until it received an additional water 

 supply from the Kivu basin, which was cut ofi'from the Nile, and 

 added to the drainage area of the big lake by the formation of a 

 volcanic dam in recent geological times. Tanganyika would thus 

 have been completely isolated and its waters more saline until an 

 outflow was established. The present effluent appears to have been 

 formed as an affluent, its bed being finally captured by a tributary 

 of the Congo. A periodic rise and fall in the lake level may 

 be caused by a temporary damming of the bed of the effluent, 

 indirectly due to irregularities in the rainfall. 



As regai'ds fauna, it is probable that at the present time all 

 the six lakes have received fairly equal investigation. Only 

 strictly aquatic animals are considered in this paper, but forms 

 obtained from the neighbourhood of a lake are included in the 

 totals. Tanganyika exhibits by far the most remarkable features, 

 containing some 402 species of which 293 (nearly 73 per cent.) 

 are endemic. Nyasa has a total second in point of size, but this 

 has been artificially swollen by extra-lacustrine records which 

 are wanting for other lakes. It has only 24 per cent, of endemic 

 forms, while Victoria Nyanza with a smaller total has a larger 

 percentage of endemics, namely 38 per cent. The three smaller 

 lakes show a great reduction both in number of types and number 

 of endemics. Tanganyika is further distinguished in that 57 out 

 of 168 genera are peculiar to its waters, whereas Nyasa can only 

 muster 6 endemic genera and the other lakes fewer still. 



The fishes of Tanganyika are of outstanding interest, comprising 

 146 species, of which 121 are endemic. The most notable feature 

 is the number and high degree of specialisation of the Cichlidie, 

 which with 27 genera (21 endemic) and 89 species (84 endemic) 

 is the richest Cichlid fauna in the world. A species of incrusting 

 gymnolsematous Polyzoon occurs, such forms being mostly marine. 

 There is a large molluscan fauna, and of the Gasteropods more 

 than two-thirds exhibit a marine-like appearance. These are 

 known as the thalassoid or halolimnic group and are without 

 exception endemic. There are no thalassoid Lamellibranchs. 

 Twelve species of prawns are known, typically fresh- water in 

 character, ))ut specialised and all peculiar to the lake. There is 

 an endemic genus of crabs, with 3 species. The Eucopepoda, 

 Branchiura, and Ostracoda are well represented, each showing a 

 large proportion of eademic species. The Cladocera are con- 

 spicuously al)sont from the lake and the Rotifera are i-elatively 

 few in number. This may be related to the salinity of the water. 

 The medusa originally described from Tanganyika has now been 



