FAUNA OF THE AFRICAN LAKES. 



601 



(p. 550). The forms in question, described and figured by 

 Brusina (58 : 59), come from fresh-water Pliocene beds in Dal- 

 matia, Croatia, and Slavonia. From this it might be argued that 

 the hikes of the Mediterranean region which existed in Tertiary 

 times were the source from which the Tanganyika Gasteropods 

 have been derived. It is not unreasonable to suggest that com- 

 munication \vp,s possible between these lakes and the region of 

 Tanganyika by way of the valley of the Nile and the Great Rift 

 Valley. While less objection can be taken to this view than to 

 Moore's, or even to the suggestion of a Cretaceous origin for the 

 molluscs, there are difficulties in accepting it as a complete 

 solution of the problem. To confirm this theory, either fossil 

 forms of like nature should be forthcoming in some intermediate 

 region, or (as a communication to the north of Tanganyika is 

 assumed) living types should be found in Lakes Kivu, Edward, or 

 Albert. Since neither are known to occur, the case for this 

 source of origin is unsupported. 



Since the shells of the thalassoid molluscs have been held to 

 resemble (1 ) mai'ine Jurassic types of the Anglo-Norman basin, 

 (2) in one instance a widely distributed fresh- or brackish-water 

 genus from the Upper Cretaceous of Hungary and North 

 America, (3) a series of fresh-water fossils from beds of Newer 

 Tertiary age in Jugo-Slavia, the evidence afforded is so contra- 

 dictory as to offer little guidance in determining the origin of the 

 fauna of Tanganyika. 



It is a very natural suggestion that the thalassoid appearance 

 of the Tanganyika Gasteropods is direct!}^ due to the size, depth, 

 and quasi-oceanic conditions prevailing m such a lake *. That is 

 to say, that a marine aspect has been produced in certain 

 members of the ordinary African fresh-water series as the result 

 of convergence. To those who have visited these lakes and 

 realised their vastness this seems plausible, but the difiiculty has 

 then to be faced, that similar forms are not forthcoming in Nyasa 

 and Victoria Nyanza. 



An hypothesis in which this view is introduced, but which has 

 other novel features, was brought before the International 

 Congress of Zoology in 1913 by Germain (87). He holds that 

 previous writers have been wrong in considering Tanganyika by 

 itself, and urges that a clearer understanding of the facts becomes 

 possible on taking into account the oi-ganisms which inhabit 

 neighbouring lakes and rivers. Dealing with the Prosobranch 

 Gasteropods, which exhibit |;6«r excellence a thalassoid facies, 

 Germain asserts that while Tanganyika contains a much larger 

 series of such forms than any other lake, it is not the sole locality 

 in which they may be found. He considers that Lakes Mwero 

 and Nyasa, as well as the Upper and Middle Congo, contain a 

 number of Prosobra.nchs (chiefly Melaniidse) the marine aspect 

 of which it is impossible to deny. Proceeding to discuss the 



* A corresponding suggestion has been made to explain the marine appearance of 

 certain Crustacea and other organisms in Lake Baikal. 



Proc. Zool. Soc— 1920, No. XL. 40 



