1901.] OF THE GREAT AFRICAN" LAKES. 407 



concerned, which I indicated as probably able to throw light upon 

 the validity of the comparison of the halolimmic molluscs with those 

 of the Jurassic seas, we see that the results of the morphological 

 examination of the halolimmic molluscs are in exact accordance 

 with such a view ; they are, in fact, exactly what we should expect 

 if the comparison be sound. 



It now remains therefore to follow out the second line of inves- 

 tigation, and to study the observations which have been made 

 during the second Tanganyika expedition respecting the Mollusca 

 which are to be found in the other great African Lakes besides Tan- 

 ganyika and ]N T yasa. I may preface this part of the enquiry with 

 the remarks, that if, according to Professor Gregory's supposition, 

 the halolinmic fauna is the relic of an ancient freshwater fauna, 

 this old fauna ought to appear in some of the other lakes besides 

 Tanganyika, or we shall be forced to suppose that it has been 

 destroyed in every other African lake by catastrophes of one sort 

 or another, a supposition which is sufficiently improbable. But 

 even supposing such a series of catastrophes to have occurred, it 

 is quite clear that the halolinmic fauna, if it is the ancient fresh- 

 water fauna of the continent, will be found in the old lake-deposits 

 which are met with all over the African interior. If it does not 

 occur in any of the other African lakes, and is not to be found in 

 the old lake-deposits, then we may dismiss the idea of its being an 

 old freshwater fauna on this account alone, as such a supposition 

 is in that case void of any evidence whatever. 



In order to examine as many lakes as possible, the second 

 Tanganyika expedition, after visiting Nyasa and Tanganyika, 

 passed northward to Lake Kivu. During this jouruey it was 

 found that Tanganyika had at one time extended far beyond its 

 present northern limit, for the floor of the great valley was com- 

 posed of old lake-deposits, containing the fossilized remains of 

 shells similar to those now living in Lake Tanganyika itself. 

 Beyond these deposits, the floor of the valley was greatly raised and 

 was found to be composed of old eruptive granitoid rock-ridges; on 

 these there were no traces of any lake-deposit ever having rested, 

 and beyond these ridges, at an altitude of four thousand eight 

 hundred and odd feet above the level of the sea, there exists Lake 

 Kivu, the outlet of which flows over the top of the ridges in 

 question into Lake Tanganyika, in the shape of the Itusisi river. 

 ]n Kivu we found no traces of the halolinmic molluscs, or indeed 

 of any of the halolinmic animals whatever, the fauna being 

 simply that of a typical tropical pond, the gaeteropodous section 

 of it being represented by the following generic forms : — 



Bythinia. 



Mi hi II i a . 



Planorbit. 

 Limneea, 



At the north end of Lake Kivu the greal valley of the lake, [g 



