604 DR. W. A. CUNNINGTOIsr ON THE 



which, as isolated areas of land, are well known to possess faunas 

 and floras largely peculiar to themselves. 



The last-mentioned hypothesis, even if satisfactory in the main, 

 makes no attempt to account for the presence of the medusa in 

 Tanganyika, and accordingly a few senlences are needed on this 

 aspect of the question. 



In dealing with the distribution of African fresh-water fishes, 

 Boulenger has discussed the problem of Tanganyika, and states 

 tliat he cannot admit Mooi'e's contentions (26, p. 422). He refers 

 to the inconchisive evidence afforded by the so-called halolimnic 

 Gasteropods, and clearly regards the medusa (now known from 

 other parts of the continent) as the only organism in the lake for 

 which it is necessary to account in any special manner. He 

 points out that palseontological evidence exists of a Middle 

 Eocene (Lutetian) sea which extended over a large area in 

 JS'orthern Africa [vide also Hudleston 103, p. 352), and suggests 

 that this would afford a rational explanation of the present 

 distribution of the medusa in Africa. 



With this view Gravier is not in agreement (90, p. 221). He 

 gives it as his opinion that the medusa may well have migrated 

 from the sea at a recent epoch, especially should it possess a 

 hydroid stage, as is held likely by Browne {cf. 56, p. 306). Its 

 present distribution in Africa he explains by reference to the 

 possibilities of intercommunication between the river systems. 



Having dealt at considerable length — as becomes its import- 

 ance—with the fa,una of Tanganyika and the views put forward 

 to account for its very unusual character, it is now possible to 

 proceed to a brief study of the remaining lake faunas. While 

 there are smaller totals and fewer peculiar types, the bigger lakes 

 at least are not devoid of interest. 



Victoria Nyanza, with 38 per cent, of endemic species, clearly 

 has characteristics of its own, these being more prominent in 

 some groups than in others. By far the most conspicuous group 

 is the Pisces, containing, as in the case of Tanganyika, the largest 

 series of foi-ms. It comprises also over half the total number of 

 endemic species and the only two endemic genera. The Mollusca 

 again are noteworthy, but in this case the Lamellibranchs are 

 more striking than the Gasteropods, exhibiting a larger pro- 

 portion of endemic types. Of the few Ostracods I'ecorded from 

 the lake, 5 out of 7 are described as peculiar, and the Oligochsete 

 worms are represented by 6 species, 4 of which are endemic. 

 Victoria is the only lake besides Tanganyika which contains the 

 medusa, though this should peiliaps be regarded as subspecifically 

 distinct. It is only in this lake that the common Hydra is 

 known to occur. Many groups are wholly without endemic 

 representatives. 



Generally speaking, Lake Nyasa exhibits very similar features, 

 but with rather fewer peculiar forms. Fishes constitute half the 

 total number of endemics, and 5 endemic genei'a out of 6. 

 A considerable number of molluscs are known, nearly half being 



