THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 169 



impression that the molluscan population of Nyassa retains its 

 character of an importation from the ponds and streams which 

 in the vast lake is, as it were, completely out of place. 



Let us now turn to Lake Tanganyika, and compare the dis- 

 tribution of the molluscs in this lake with the observations I 

 have just described in relation to Nyassa. But let me first 

 point out that the physiographical features of Lake Tanganyika 

 are slightly diflPerent from those obtaining in the great Nyassan 

 valleys. Tanganyika is 2700 feet instead of 1500 feet above 

 the level of the sea; but notwithstanding this greater eleva- 

 tion, the climate of Tanganyika is appreciably hotter than that 

 of Nyassa. 



The shores of Tanganyika are, perhaps, on the whole more 

 precipitous, and are certainly less extensively fringed with the 

 broad coast belts of modern alluvium so characteristic of the 

 margin of Nyassa. The southern half of Lake Tanganyika is 

 not nearly so deep as Nyassa, the water being generally little 

 more than from 900 to 1200 feet in depth; and it is not so 

 pure, being always impregnated with an appreciable taint of 

 several salts. 



These slight physiographical diflFerences which exist between 

 Nyassa and Tanganyika cannot, however, be considered as 

 having any potency to modify the individual fauna which the 

 lakes contain. For there is a far greater difference between 

 the physiographical features of Nyassa and the Victoria 

 Nyanza than between Nyassa and Tanganyika. Yet the 

 molluscan population of both the Victoria Nyanza and Nyassa 

 are essentially the same. We cannot, therefore, regard the less 

 amount of difference which is perceptible between Nyassa and 

 Tanganyika as in any way responsible for the wide faunistic 

 differences which exist between these lakes. This view is also 

 in accordance with the very important fact that all the genera 

 found in Nyassa exist in Tanganyika also. There is no more 

 difference between the normal, non-Halolimnic molluscan popu- 

 lation of Tanganyika and Nyassa than there is between that of 

 Nyassa and Victoria Nyanza — a state of things which seems 

 to indicate clearly that the difference in physiographical 



