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 polypterus 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 
