1901.J OF TITE GREAT AFRICAN LAKES. 469 



expedition crossed the Mau Plateau and again descended into the 

 lesser and more eastern series of faulted valleys in which Lakes 

 Baringo, Naivasha, Rudolf, and several minor pools and marshes 

 occur. The fauna of Lake Baringo had already been examined by 

 Dr. Gregory, and the molluscan fauna in this and the minor lakes 

 which occur in the same district are similar in kind to those which 

 are to be found in the Victoria Nyanza itself. So also with respect 

 to the larger lakes Stephanie and Rudolf, which lie to the north, 

 we have now the evidence of various explorers which makes it 

 quite certain that nothing of the nature of the remarkable halo- 

 limnic Gasteropods occurs in either of them. 



To sum up, all the evidence which has recently been collected 

 with respect to the following lakes — Shinva, Xyasa, Rukwa, Ban- 

 gweolo, Moero, Tanganyika, Kivu, the Albert Edward Nyanza, the 

 Albert Nyanza, the Victoria Nyanza, Lake Baringo, and Lake 

 Rudolf — declares in the most emphatic manner that the halolimnic 

 Gasteropods, and. indeed, the whole halolimnic fauna, is found no- 

 where beyond the coufines of Lake Tanganyika itself, nor have the 

 prolonged and tedious examinations which I have made among the 

 numerous old lake-deposits, like those which occur at a considerable 

 elevation north and west of Xyasa, all about Lake Kivu, on the 

 plains between Kivu and the Albert Edward Nyanza, in the Semliki 

 Valley, and in the neighbourhood of the Albert Nyanza, revealed 

 any traces whatever of the past existence of these forms. On the 

 other hand, a very superficial examination of the old lake-beds 

 which have been at one time part of Tanganyika itself, like those 

 v hich occur near the outlet of the lake, at Masswa on the west coast 

 and beyond its northern extremities, is all that is needed to 

 demonstrate the former presenceof these forms in abundance in such 

 districts. I think therefore that I am now justified in putting it 

 forward as a demonstrated couclusiou, that the halolimnic fauna 

 neither is, nor ever has been, present in any of the districts sur- 

 rounding the numerous and wide!}' separated lakes which I have 

 just named. The recently accumulated facts regarding the dis- 

 tribution of the African freshwater mollusca show that there is in 

 the lakes which now exist, and has been in those lakes and rivers 

 which have long since vanished, a fauna similar to that which now 

 generally survives; and thus the evidence which the facts of 

 distribution reveal is in no way opposed to the view' that the 

 halolimnic fauna is something exotic which has been added to 

 the ordinary freshwater fauna in the case of Tanganyika. It 

 offers no support whatever to the view that the halolimnic fauna 

 is the relic of an old freshwater stock. On the other hand, the 

 fad 9 directly militate against this view, u it 1 1 the cumulal ive force 

 of negative appearances. They are in qo way opposed to our accept- 

 ance of the clue which is afforded as to the nature and origin of 

 the halolimnic fauna by the similarity of the halolimnic shells 



to those which occur in the Jurassic Seas. On the other hand, 

 they are exactly in accord with such a view. 



