Connection of Lake JanganyiJca ivith the Sea. 453 



sibly the pelagic Protozoa, are much more nearly related to the 

 similar marine organisms which have repeatedly contaminated the' 

 fresh waters of the world elsewhere. It should, however, be clearly 

 understood that even these apparently more normal types have not 

 been found in Nyassa, Shirwa, or Kela, nor have they been recorded 

 from any of the Great Lakes further north. Therefore, although they 

 are less peculiar than their associates in Tanganyika, they probably 

 belong to the same quasi-marine, or what I shall in future call the 

 Halolimnic group. 



The results of a systematic survey which was made of the 

 geographical and bathymetric distribution of the aquatic molluscs 

 throughout the wide area over which the expedition had to pass have 

 demonstrated in the most conclusive manner the complete duality of 

 the Tanganyika fauna as a whole.* In Nyassa, Shirwa, Kela, and 

 several minor lakes, taken together, all of which I visited and 

 dredged, there have been found the following molluscous types : — 

 Unio, Spatha, Corbicula, Iridina, Limncea, Isodora, Pliysopsis, 

 Planorbis, Ancylus, Ampullaria, Lanistes, Vivipara, Cleopatra, Bithynia, 

 and Melania. 



Not all of these fifteen genera which are now found living in 

 Nyassa are present in the smaller lakes. In the Shirwa they are 

 reduced to five, and in Kela they are only three. The full Nyassan 

 series has, however, been recorded from the Victoria Nyanza, and in 

 this more northern group of lakes there is again seen the curious 

 reduction in the number of genera as we pass from the greater lakes 

 to the less. Prom these facts of distribution it is apparent that the 

 genera of molluscs, which occur in the African fresh waters, are very 

 constant over an immense area of ground. There can indeed be little 

 doubt that the genera found in Nyassa characterise and constitute 

 the type of the truly African fresh- water fauna as a whole. 



The fauna of Tanganyika appears therefore to form a striking 

 contrast to this rule of uniformity in type which characterises the 

 fauna of all the other lakes. Such divergence is, however, in one 

 sense more illusory than real. All the Nyassa or Victoria Nyanza 

 genera are found living in Tanganyika, and the fauna of this lake 

 does not differ from the faunas of the others in kind or as a whole. 

 It differs from them merely in there being here added to the normal 

 series a number of molluscs which are not found elsewhere. To this 

 superadded list, however, there attaches a unique interest, as it is 

 entirely composed of those ten genera of gasteropods and lamelli- 

 branchs which were instanced as possessing the same marine appear- 

 ance as the jelly-fish and prawns. 



The strange geographical isolation of the halolimnic molluscs which 



* The full details of my observations on the distribution of the Halolimnic 

 molluscs are now in the hands of the Editor of the ' Quart. Journ, Micr. Sci.' 



