THE FAUNA OF THE GREAT AFRICAN LAKES. 637 



between ancient and living fresh-water shells, the anatomical 

 characters even of the latter not being then known, the idea 

 has been promulgated that the whole of the isolated group 

 in Tanganyika has had no marine origin but represents a 

 sort of prehistoric condition of the African lake faunas in 

 general. There are, however, to my mind several fatal 

 objections to such a view, first and foremost among which 

 is the total isolation of the Tanganyika group to the lake 

 in which it now lives. If this group were really represen- 

 tative of the ancient African fresh-water Cretaceous stock, 

 more or fewer of the remaining lakes would almost certainly 

 have contained representatives of this group, and although 

 it has been argued that change in the salinity of the water, 

 of which there is no proof, or some other conditions not 

 specified may have swept the old faunas out of the remain- 

 ing lakes, it is to my mind in the highest degree improbable 

 that such causes would ever have exterminated the animals 

 in question out of all the lakes but one. Then again the 

 gastropods are associated with jelly-fish, animals which 

 have always been regarded as typically marine, and to 

 argue that in the past times jelly-fish were fresh- water 

 organisms, because certain enigmatical shells in a lake 

 appear to correspond with fresh-water cretaceous forms, is 

 to my mind to make a gratuitous assumption which the facts 

 of the case do not in the least necessitate. Lastly the 

 Paramelanias themselves although presenting in the char- 

 acters of their shells, such a marked similarity to the 

 Pyrguliferas of the chalk, are quite as much like some of 

 the marine jurrassic Purpurinas, and in this way the 

 Palajontological evidence upon the subject can be made to 

 cut both ways. Moreover, supposing the Paramelanias 

 to be really the living representatives of the Cretaceous 

 Pyrguliferas as some of the latter were estuarine forms, it 

 is quite likely that they may have been existing in or 

 about Lake Tanganyika, if this lake was in connection 

 with the sea, and have lived on in the lake along with those 

 marine animals which were able to stand a succession of 

 later changes. 



The palaiontological evidence touching the origin of the 



