548 DR. W. A. OUNNINGTON ON THE 



genera Tiphohia and Bathanalia, which are among the most 

 striking of the thalassoid forms from Tanganyika. At present 

 this remains the only example of an endemic family characteristic 

 of that lake. 



The representatives of the three remaining families are all 

 Avell-known fresh-water genera, and do not offer much of interest 

 on which to comment. Of the Limnfeidse, specimens are on 

 I'ecord only from the three large lakes and Lake Edward, which 

 is rather surprising as the group has a wide distribution in 

 Tropical Africa. Limncea natalensis is the most widely distributed 

 species, being known from Tanganyika, Victoria, and Nyasa, as 

 well as from Natal. The families Planorbidse and Ancylidfe are 

 each represented by the type-genus only. Thirteen species of 

 Planorhis are enumerated, of which Tanganyika contains the 

 largest series, viz. 7 forms, 4 of which are endemic. In Nyasa, 

 it is strange to note, only a single widely distributed species 

 has been found. The form of commonest distribution in the 

 lakes is P. sudanicihs, which occurs in four of them as well as in 

 the River Nile. Turning to the Ancylidfe, it will be seen that 

 ail endemic species -of Ancylus is on i-ecord both in Tanganyika 

 and in Lake Victoria. No specimens have been observed in the 

 other la,kes, but they may have been overlooked owing to their 

 small size. 



It is not unreasonable to i-efer here to the suggestion already 

 cited in the section dealing with the fishes (c/. p. 535) that the 

 number of different animal types living in a lake is directly 

 proportional to its size. The figures for the Gasteropoda are 

 sufficiently large to afford some evidence for this contention, and 

 they are accordingly given in the following table, which records 

 the number of families and genera, as well as species : — 



Tangan- Victoria ^t Albert Edward t^. 



•1 XT JNvasa. -vr -m- Kivu. 



yika. JSJyanza. ■' JNyaiiza. JNyaiiza. 



Number of Species 84 



„ Genera 35 



,, Families 8 



It will be seen that Tanganyika, with its admittedly unique 

 fauna, is in a category by itself, but the remaining lakes, which 

 are arranged in order of size, form a series with decreasing totals, 

 and thus support the general truth of the assertion. 



The distribution of the Gasteropoda in the lakes may be sum- 

 marised in a few sentences. Tanganyika,, containing 84 species, 

 of which 76 are endemic, is altogether exceptional and is prin- 

 cipally characterised by a lemarkable series of marine-like or 

 thalassoid forms. While the latter are not CQufined to one 



