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 



Spckia zonata, left, compared with an example of the Jurassic genus 

 Neridomits, 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 palseontological 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 



