FAUNA OF TflB AFRICAN LAKES. 509 



obtain specimens of tliese medusfe propei'ly prepared for scientific 

 investigation. This valuable material, on its arrival in England 

 at the end of 1892, was placed for examination in the hands of 

 R. T. Giinther of Oxford. 



It was Giintlier who first put forward a, detailed theory to 

 account for the presence in a central African la,ke of animals 

 with apparently marine affinities. In his report on the Tan- 

 ganyika medusa he suggested (95, p. 289) that if at some early 

 period the interior of Africa stood at a much lower level, with 

 the Atlantic Ocean extending over the Congo basin and the lake 

 existing as an arm of the sea,, it would then be possible i-eadily 

 to explain the presence in it both of the medusa and the remark- 

 able molluscan forms. As the lake became separated from the 

 sea, so the salinity of the water would tend to decrease if it were 

 supplied with inflowing rivers and found an outlet tlirough which 

 to carry awaj the salt. 



Since the problem presented by Tanganyika had now become 

 so intei-esting and definite, it seemed very desirable that the lake 

 fauna should be investigated upon the spot by some trained 

 zoologist. The matter was taken up by Ray Lankester, who, 

 with the help of others' interested in the question, obtained the 

 necessary grants from the Royal Society and organised the First 

 Tanganyika Expedition. This was put in charge of J. E. S. 

 Moore of the Royal College of Science, who from that time 

 onwards contributed largely to the knowledge of the lakes of 

 Central Africa. 



The expedition left England in the autumn of 1895, and Lake 

 Shirwa was visited as well as Nyasa, while Moore spent several 

 months on the shores of Tanganyika. As a result of this journey, 

 the original problem seetned far from being solved, for the expedi- 

 tion brought matters to light which only made the lake appeal 

 more remarkable. Many strange and unique animal forms were 

 collected, certain of which were thought to show marine affinities, 

 while the majority were new to science. The collection included 

 a number of types not unexpected in a tropical lake ; only a 

 proportion of the fauna exhibiting the imusual features to which 

 reference has been made. Moieover, although the expedition had 

 visited Nyasa and Shirwa, in neither of these lakes had marine- 

 like forms been found resembling those of Tanganyika. 



Through a study of the newly acquired materia], especially the 

 anatomy of certain of the molluscs, Moore arrived at the conclu- 

 sion that the forms exhibiting a marine appearance could not be 

 compared directly with any marine types of the present day, but 

 rather possessed archaic and generalised characters which sug- 

 gested that they were the relics of a marine fauna of a previous 

 age. Further, he considered that some of the Tanganyika shells 

 ofiered a strong resemblance to certain marine Jurassic fossils, in 

 particular those of the Inferior Oolite of the Anglo-Norman 

 basin. Taking this as evidence of the period from which these 

 remarkable living forms had been derived, Moore propounded 



