THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 165 



comprise among others the six genera of Gasteropods which 

 have been known hitherto only by their empty shells, namely, 

 Ty phobia, Paramelania, the so-called Lythoglyphus of 

 Tanganyika, Syrnolopsis, Nasopsis, and Limnotrochus. 

 I was, during my recent expedition, enabled to add to this 

 isolated series at least two entirely generic forms, for which 

 I have proposed the names Bathanalia and Bythoceras. 

 We have, therefore, now in Tanganyika some eight genera of 

 Gasteropods which are not found in any of the other lakes, 

 and to this isolated list of molluscs there should probably be 

 further added among the Lamellibranchiata the so-called 

 Unio Burtoni, and one of the Tanganyika Spathas. Con- 

 sequently there are now known to exist in Tanganyika ten 

 genera of molluscs, which appear to be restricted to the lake 

 in which they were originally found. 



Although I am concerned here primarily with the dis- 

 tribution of the molluscs in these lakes, it must be clearly 

 understood that the marine organisms, such as jelly-fish, crabs, 

 prawns, sponges, and Protozoa, with which the above molluscs 

 are associated, share equally the same geographical limitation. 

 The ten quasi-marine molluscan genera being, in fact, only one 

 section of a complete fauna, containing widely separated types, 

 which in Tanganyika exists along with the normal fresh-water 

 stock the lake contains. The fauna of Tanganyika is thus a 

 double series, and to distinguish its apparently marine con- 

 stituents from the more normal lake animals I shall speak of 

 them in future as the Halolimnic group. 



Now it can be confidently affirmed that there are no Halo- 

 limnic animals in Nyassa, Shirwa, or Kela, all of which I 

 visited and dredged ; and they are certainly not present in 

 Bangweolo or Mwero. Yet these organisms are so con- 

 spicuous and common, when they do occur, that they would 

 certainly have been recorded from the Victoria Nyanza, the 

 Albert Edward, and the Albert, if any Halolimnic animals 

 had existed in these more northern lakes. So far as is at 

 present known, then, the Halolimnic fauna is entirely re- 

 stricted to the confines of Tanganyika, in which lake it was 



