548 



DR. W. A. CUNNINGTON ON THE 



genera Tiphobia and Bathanalia, which are among the most 

 striking of the thalassoid fornis from Tanganyika. At present 

 this remains the only example of an endem.ic family characteristic 

 of that lake. 



The representatives of the three remaining families are all 

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

 on which to comment. Of the Limnceida?, specimens are on 

 record only from the thi-ee large lakes and Lake Edward, which 

 is rather surprising as the gronp hns a wide distribution in 

 Tropical Africa. Lwinmi luitalensis is the most w^idely distributed 

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

 well as from Natal. The families Pla.norbid?e and Ancylidse 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. sudanicus. which occurs in four of them as well as in 

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

 an endemic species of Ancyhis is on record both in Tanganjdka 

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

 other lakes, but they may have been overlooked owing to their 

 small size. 



It is not unreasonable to refer here to the suggestion already 

 cited in the section dealing with the fishes {cf. 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- 

 yika. 



Victoria ^ 

 Nyanza. ^ 



Albert 

 ' Nyanza. 



Edward -rr- 

 Nyanza. K^^*^' 



Number of Species 



84 



28 24 



13 



10 2 



., Genera , . , 



35 



11 9 



6 



6 2 



„ Families , , 



8 



7 6 



5 



6 2 



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 i-emarkable series of marine-like or 

 thakssoid forms. While the latter are not confined to one 



