LAKE TANGANYIKA AN OLD JURASSIC SEA. 319 



lultilled its purpose if it acts merely as a sort of couuterpoise 

 to the altogether disproportionate iraportauce which has been 

 attached to the apparent similarity between the Paramelania 

 of Lake Tanganyika and the Pyrgulifera of the upper chalk. 



Whatever opinion those competent to judge may form of the 

 comparisons which I have just instituted between the marine 

 Jurassic and the halolimnic faunas, it is obvious that these com- 

 parisons are nothing like so rash an undertaking as that 

 attempted by the three authors I have named. The view that 

 tlie Tanganyika fauna corresponds to a fresh-water cretaceous 

 stock rests on nothing but the similarity of a single type of 

 shell common to the halolimnic and cretaceous fresh-water 

 series ; and, as we have seen, the possibility of even this single 

 point of similarity being due to anything more than mere con- 

 vergence of external form has been rendered so extremely 

 doubtful by the more extended observations which one of 

 the authors named has already made^ that any attempts 

 to pursue the question further would be simply waste of 

 time. Even if the far more weighty evidence for the cor- 

 respondence of the halolimnic fauna with that of the Jurassic 

 seas rested solely on the similarity of their respective shells, 

 although such evidence would be as good as that forthcoming 

 for many sweeping geological deductions, I should, for my part, 

 be highly sceptical that it afforded any trustworthy indication 

 that the hypothesis is true. When, however, we view the 

 supplementary facts of this comparison, when we regard it in 

 the light of what I have ascertained respecting the distribution, 

 and especially the comparative morphology of the halolimnic 

 forms, it is very clearly apparent that the theory of their 

 similarity is not without much collateral support. 



We know now that the morphological characters of the 

 halolimnic fauna are those of an early oceanic stock, that they 

 do not stand midway between the living fresh-water faunas and 

 their marine beginnings, for they do not foreshadow any known 

 fresh-water types ; on the other hand, we have seen that they 

 do very distinctly foreshadow many living oceanic types, each 

 individually uniting the characters of several modern oceanic 



