LAKE TANGANYIKA — AN OLD JURASSIC SEA. 309 



times. The lake is now 2700 feet above the level of the sea^ 

 and is more than 700 miles from any coast ; there is but one 

 effluent, and the course of this river is beset with rapids and 

 with falls even long before it reaches the lower channels of the 

 Congo on its way towards the sea ; and finally^ there are no 

 true representatives of the halolimnic fauna, except, perhaps, 

 the universal Xenophoridae, in that part of the Atlantic into 

 which the Congo flows. 



The physiographical features of the continent point directly, 

 therefore, to the conclusion that the halolimnic fauna must be 

 very old. It must have been left in the great valley of 

 Tanganyika long before that part of the continent had at- 

 tained its present altitude, and when the surface of the water 

 was approximately at the level of the sea. In exact conformity 

 with this indication, it will have been seen that the halolimnic 

 animals as they now exist, although closely allied to different 

 marine genera, are not exactly similar to any oceanic species 

 that we know ; and finally, it has been shown that the halolimnic 

 Gasteropods, at any rate, stand in the relationship of immediate 

 ancestors to several of our well-known oceanic forms. There 

 thus exists evidence which appears to be practically conclusive 

 that the halolimnic animals retain the facies of a sea fauna 

 that has elsewhere disappeared, and consequently, unless they 

 have become modified out of all semblance to their original 

 marine progenitors, it is only natural to expect that on some 

 marine fossiliferous horizon we shall again encounter in a 

 fossilised condition similar molluscan shells. The hope that we 

 may in this way be able actually to " locate ^^ the halolimnic 

 fauna of Lake Tanganyika with that particular marine stock 

 from which it sprang is all the greater, on account of the very 

 striking facies which the shells of the molluscs belonging to it 

 invariably present. But in actually searching among marine 

 deposits for the particular sea fauna to which the halolimnic 

 animals may correspond, it is essential that we bear in mind 

 the caution that single comparisons are likely to be of little 

 service as afl^ording any indication that two such faunas are 

 the same. There must be in the old stock, to which we are 



