164 J. E. S. MOORE. 



lakes indeed. In the Victoria Nyanza all the Nyassan 

 genera have been recorded, and more or fewer of the con- 

 stituents of the same Nyassa list constitute the faunas of the 

 remaining members of this more northern group of lakes, — as, 

 for example, the Albert Nyanza, the Albert Edward, Beringo, 

 and the like. 



To facilitate comparison I have arranged the names of all 

 the lakes about which anything definite is known in the tabular 

 form given on p. 166. On the left-hand side will be found a 

 list of all the genera hitherto known to be contained in each. 

 From this table it will be seen that more or fewer members of 

 the Nyassa list of genera are contained in every lake, but that 

 there is a curious reduction of the number of the genera as we 

 pass from the greater lakes to the less. This is probably due 

 to the impermanence of the conditions in the smaller lakes, 

 for we find in Shirwa, which is salt, and in Kela, which has 

 periodically dried up, only those forms which can stand a wide 

 amount of change. If we pass momentarily, however, from 

 the study of the genera among the lakes to that of the specific 

 forms, it will be found that there is a certain amount of 

 variation in the specific representation in the genera contained ; 

 and that when the lakes are widely separated — as, for example, 

 Nyassa and Lake Mwero — such specific variations are often 

 strongly marked. Judged, however, by the genera alone, it 

 will be seen that there is a remarkable uniformity in the 

 character of the African fresh-water molluscs over an immense 

 area of ground. To this rule of uniformity in type which 

 characterises the molluscan fauna of all the lakes about which 

 anything is known, Tanganyika seems to form a solitary and 

 striking exception. But the differences which this lake pre- 

 sents are in one sense more illusory than real, for on inspec- 

 tion of the table it will be seen that Tanganyika does contain, 

 and fully represented, the great lake list of molluscs found in 

 the Nyassa to the south, and the Victoria Nyanza to the 

 north. It differs from the other lakes in there being here 

 added to the otherwise universal list a number of entirely new 

 forms. The genera which compose this superadded series 



