THE MOLLUSCS OP THE GEBAT AFRICAN LAKES. 173 



This generalised African lake fauua contains only those families 

 and genera of molluscs which would be regarded as typically 

 fresh-water, lake, river, and pond dwellers, in whatever con- 

 tinent the fresh water might occur. In one African lake, 

 however, but in one lake only, there have been found to exist, 

 superadded to this normal lacustrine stock, a number of 

 Gastropods which do not closely resemble any other forms 

 either living or extinct; these molluscs are also completely 

 dissociated from the remaining normal series of the lake in which 

 they occur by their modes of life. Together these molluscs 

 constitute the moUuscan section of a whole faunistic series, 

 which in Tanganyika is added to the normal fresh-water 

 stock the lake contains. This fauna forms what I have called 

 the Halolimnic group, and the tout ensemble of all the 

 Halolimnic genera is marine. 



To account for the presence of the Halolimnic organisms 

 in Tanganyika, only three hypotheses which are even tempo- 

 rarily tenable can be found. It may be supposed — 



1. That they have arisen as modifications of the ordinary 

 fresh-water fauua through prolonged isolation in the lake ; 



2. That they are the surviving representatives of an extinct 

 fresh-water stock ; or — 



3. That they are comparatively recent importations from 

 the sea. 



Let us examine each of these three possible explanations 

 in the light of the new facts of distribution which have just 

 been detailed. Unless the conditions affecting the fauna of 

 Tanganyika have been permanent for a greater period of 

 time than has been the case with any of the other lakes, they 

 could not have produced the Halolimnic fauna which this lake 

 now presents. Unless we make the further suppositions (1) that 

 the conditions in Tanganyika have been permanent, while those 

 affecting the fauna in all the other lakes have changed so much 

 as to kill off the Halolimnic forms they once possessed ; or 

 (3) that all the other lakes are much younger than Tanganyika, 

 and that therefore the Halolimnic fauna has not had time to 

 develop in them yet. There is no evidence for either of these 



