THE TANGANYIKA PROBLEM. 
35i 
very surprising fact that every one of the halolimnic gas- 
tropod genera should contain one or more forms which are 
indistinguishable from corresponding Jurassic types, or, in 
other words, that the gastropodean section of the halo- 
limnic fauna of Tanganyika should correspond en bloc 
with the gastropodean remains left by the Jurassic seas. 
We have, in fact, here something which is obvious and 
tangible, and which, at any rate, on the face of it looks 
Spekia zonatci, left, compared with an example of the Jurassic genus 
Neridomus , on the right. 
as if it would lead directly to the solution of the whole 
Tanganyika problem with which we have been con- 
cerned. What it is requisite to ascertain further is 
whether, under the circumstances, such a comparison is 
justifiable, or, in other words, are there any geological 
or paleeontological considerations which will render it im- 
possible, or, at any rate, extremely unlikely that in Tan- 
ganyika we should encounter Jurassic forms persisting to 
the present day. So far as I can ascertain, there is no 
