634 



SCIEXCE PROGRESS. 



The isolated group of Molluscs constituted by these seven 

 or eight genera are. then, something- superadded to the 

 general African lake fauna, and all the members of this 

 group seem to be strictly limited to the confines of Lake 

 Tanganyika alone. The evidence on this last point, how- 

 ever, is. as will be seen from the appended table, not vet 

 complete, but it is, I am convinced, sufficiently complete to 

 show that this group of Tanganyika Molluscs is not repre- 

 sented in any of the lakes in which Molluscs have hitherto 

 been recorded. They are certainly not present in Xyassa, 

 Shirwa, Bangweolo. or Kela ; and the members of the 

 above group are so conspicuous and common when they do 

 occur, that they would certainly have been recorded from 

 the\ ictoria Xyanza and the other members of the northern 

 group of lakes if they had existed there. 



Gastropod Genera 



occnrring in 

 the African Lakes. 



Isidora 



Physophisis - 



Planorbis 



Ampullaria - 



I^anistes 



Vivipara 



Cleopatra 



Bithynia 



Melania 



-luma - 

 riania 



L)-thogl\-phus 

 Lymnotrochus 

 Undescribed Genera) 

 in Tanganyika j 



Table showing the distribution of Gastropod Genera in the 

 principal African Lakes. 



There is, then, not a shadow of evidence to support the 

 idea that the remarkable forms of Molluscs recorded from 

 Tanganyika are a characteristic feature of the African lake 



