THE MOLLUSCS OP THE GREAT AFRICAN LAKES. 168 



Sir Roderick Murchison in 1852. Yet such a view is now 

 supported by the strongest kind of zoological evidence it is 

 possible to get. 



In order to arrive at a sufficiently complete comprehension 

 of the general type of the molluscau fauna wliicli characterises 

 the individual lakes, it is quite unnecessary and even prejudicial 

 to discuss the question of the occurrence or non-occurrence of 

 many of the so-called specific forms, since a large number of 

 these are merely geographical varieties, while others have 

 been based on minute and often purely fanciful conchological 

 distinctions. The four species of Hylacantha, described 

 by Bourguignat in 1890, for example, are certainly nothing 

 more than the rather remarkable polymorphs of the original 

 Typhobia Horei described by Smith in 1881. But what is 

 true of the polymorphic Typhobia is equally true of the 

 polymorphic Paranielania and Neothauma. I shall there- 

 fore consider the distribution of the genera alone, or the main 

 issue will become lost in the pursuit of really non-existent types. 



In Nyassa, which was the first great lake I reached, 

 and which has hitherto been better known than all the rest, 

 there have been recorded some sixteen genera of molluscs, 

 namely,Limnaea, Isodora,, Physa, Physopsis, Planorbis, 

 Ancylus, Ampullaria, Lanistes, Vivipara, Cleopatra, 

 Bythinia, Melania, Spatha, Iridina, Corbicula, and 

 Unio. 



In the smaller lakes occurring in this district, such as 

 Shirwa, there have been found a fewer number of the same 

 genera. In Kela, which is within twenty miles of the south 

 end of Lake Tanganyika, I found Planorbis and Limn sea. 

 In Mwero there have been recorded Unio, Ampullaria, 

 Lanistes, Vivipara, Cleopatra, Bythinia, and Melania ; 

 while in Bangweolo, according to Lieutenant Weathei'ly, there 

 are no shelled molluscs, but it is hardly credible that their 

 absence in this lake will be maintained. It is thus certain 

 that the generic forms occurring in Nyassa completely 

 cover the raollusca in a very large number of African 

 1 ' Auu. des Sci. Nat.,' septieme serie, ix, x, 126, 1890. 



