298 NATURAL SCIENCE [May 



The Mollusca of Lake Tanganyika 



Since his return from Africa, Mr J. E. S. Moore has been hard at 

 work examining the animals, especially the mollusca, brought back 

 by him from Lake Tanganyika. It has already been announced 

 that the fauna of this lake furnishes evidence for its connection with 

 the sea at no very distant date, geologically speaking. On January 

 27th Mr Moore read before the Koyal Society a paper on this sub- 

 ject, of which the following extract has been issued : — 



" The results of the morphological examination of the animals 

 obtained have made it evident that the fauna of Lake Tanganyika 

 must be regarded as a double series, each half of which is entirely 

 distinct in origin and nature from the other. The remarkable 

 molluscan shells which were brought home by Burton and Speke, 

 form but a small part of the molluscan section of the more abnormal 

 of these fresh-water stocks. Besides molluscs, the lake was found 

 to contain fishes, Crustacea, Coelenterata, and Protozoa, all of which, 

 like Speke's shells, present the most curious marine affinities ; and 

 for distinctive purposes the individual members of this unique 

 assemblage of quasi-marine fresh-water organisms are described as 

 members of the Halolimnic group. 



"The distribution of the aquatic faunas occurring in Lakes 

 Shirwa, Nyanza, Kela, and Tanganyika, all of which were visited 

 and dredged during the expedition, shows (together with what is 

 already known respecting the Victoria Nyanza and the more northern 

 lakes) that the Halolimnic animals are exclusively restricted to 

 Tanganyika. It is thus rendered inconceivable that the Halolimnic 

 forms can have arisen through the effect of ordinary conditions 

 operating upon the population which the lake originally possessed. 

 For the same reasons, it becomes equally clear that the Halolimnic 

 animals cannot be regarded as the survivors of an old fresh-water 

 stock. Since, if we accept either of these suppositions, we are bound 

 by the facts of distribution to believe also that the Halolimnic 

 animals have been destroyed in every African lake but one ; a sup- 

 position which may be ingenious, but which, when the number of lakes 

 existing in the African interior is fully realised, becomes grotesque. 



" Apart from the physical difficulties which the present effluent 

 of Tanganyika presents to the ingress of organisms from the sea, it 

 is impossible to regard the Halolimnic forms as having recently 

 transmigrated thither from the ocean, since none of these animals 

 is exactly similar to any marine organism at present known. 

 They must, therefore, have been in Tanganyika long enough to be 

 moditied into their present condition from the living oceanic species 

 which we know, or they retain the characters of a sea-fauna that 

 has elsewhere become extinct. 



