THE TANGANYIKA PROBLEM. 353 



we could still regard it as a somewhat surprising coincidence 

 if two or three modern types had done the same ; but when, 

 as in the case of the halolimnic shells, we find that more than 

 half a dozen genera, all the halolimnic genera in fact, contain 

 types which are indistinguishable from marine Jurassic 

 forms, the likelihood of such a correspondence being a mere 

 coincidence becomes improbable in the extreme. There is 

 thus in this comparison direct and weighty evidence as to 

 what the halolimnic fauna really is, and it remains for us to 

 consider whether the character of the halolimnic animals 



The Melania admirabilis of Lake Tanganyika. 



other than the gastropods can be readily fitted in with this 

 somewhat startling indication that Tanganyika practically 

 represents an old Jurassic sea. The facts upon this head 

 are somewhat meagre, but they are certainly not without 

 significance. Thus the ganoid polyptcrus at least had 

 allies in the Jurassic seas. Most of the teleostian fishes 

 appear to have arisen later, but they probably had emerged 

 about that time, and we obviously need not suppose that 

 the lake would become suddenly cut off from a western 

 ocean. The prawns, crabs and jelly-fishes of the halolimnic 

 group are all forms which might very well have belonged 

 to a Jurassic series, while the sponge Potamelepus is not 



