THE FAUNA OF THE GREAT AFRICAN LAKES. 633 



entirely lacustrine character. But what is true of the 

 Gastropods in this respect is true of the remaining- orders 

 and families. The lake contains those only which are usually 

 found to be associated together in the fresh waters of all the 

 other great continents. There are individual peculiarities in 

 some of the species contained in these Nyassan genera which 

 are quite sufficient to distinguish the Nyassan fauna from 

 that of any of the other African lakes. But they are by no 

 means so great as might have been expected if the diverse 

 conditions which I have pointed out as obtaining in the 

 different lakes have really been operating for a very pro- 

 longed period of time. Thus the character of the fauna of 

 Nyassa is entirely that of an ordinary lacustrine series, and 

 has undoubtedly been derived from a purely fresh water 

 stock. There is no form known in this lake wdiich has any 

 marine attributes about it, and there is nothing to suggest 

 in the least that the lake has at any time been in connection 

 with the sea. 



The fauna of Nyassa is, however, extremely important 

 to the present inquiry in another way. The Mollusca found 

 in this lake, together with the fish, Crustacea, and so on, 

 compose a list of genera which includes all those found in 

 almost all the neighbouring lakes, and thus if we arrange 

 the names of the Nyassan genera of Gastropods, for ex- 

 ample, in a vertical column, and write the names of the 

 principal African lakes horizontally along the top, it will be 

 seen that the genera of Gastropods which have been found 

 to inhabit the other lakes are those which are contained in 

 the Nyassan list, more or fewer of them being represented 

 as the particular case may be. Thus in Shirwa we have 

 Vivipera, in Kela, Planorbis, and so on. In the Victoria 

 Nyanza and the Albert Nyanza there are a larger number 

 of these same Nyassan genera, and it will be seen at once 

 from the table that the Nyassa list covers all those hitherto 

 found in every African lake but one. This solitary excep- 

 tion — Lake Tanganyika — does, however, contain the Nyassa 

 list in full, but above and beyond this list there are some 

 seven or eight genera of Gastropods which, so far as is at 

 present known, are found living nowhere else in the world. 



