1901.] ©F THE GllEAT AFUICAJT LAKES. 463 



organisms ol Lake Tanganyika, or forms closely allied to them, 

 would be found in these lakes also. He supported this view in a 

 discussion which followed the reading of a paper of mine at the 

 Eoyal Society, by the fact that the geologists White ^ and Tausch^ 

 had obser\ed that the shells of the Paramelania of Tanganyika 

 were very similar to some fossil forms wdiich occur in the Upper 

 Cretaceous freshwater beds of North America and Southern 

 Europe. It was from this single observation that Professor Gregory, 

 Mith a certain rashness, drew the inference that the halolimnic 

 Gasteropods of Tanganyika we.vQ the remains of an old freshwater 

 fauna, once widely distributed in Africa, and still to be found in 

 the more permanent great lakes. In opposition to this view, I 

 pointed out that any conclusions respecting the similarity of a living 

 and extinct fauna which were based upon a single conchological 

 correspondence were so very much open to question, that zoologists 

 in general would not accept them, whatever paleontologists might 

 feel inclined to do ; and I shoN^ed later that the comparisons 

 of White and Tausch are thrown complel^ely into the shade and 

 annulled by a much more striking comparison which can be 

 made between the whole of the halolimnic genera (with the 

 exception of Bythoceras) and the shells which have become 

 fossilized among the remains of the Jurassic seas. Whether the 

 conchological comparison of living with extinct forms is in any 

 case justifiable is a matter upon which I have at present no 

 opinion ; but I wish to make it clear that whatever force there 

 may have been in the first comparison between one of the 

 Tanganyika shells and a certain freshwater Cretaceous form is 

 deficient when matched with the comparison between the long 

 series of halolimnic Gasteropods and a corresponding number of 

 the shells of those species which occur in the Jurassic Seas. In 

 this comparison, we have evidence touching the origin and nature 

 of the halolimnic fauna which the paleontologists at any rate 

 must regard as weighty; for if it is not, then very many existing 

 paleontological determinations, which rest upon a similar com- 

 parison of shells, would also be worthless. 



The evidence which we require to throw further light on this 

 most interesting matter lies along two distinct lines of investi- 

 gation. AV^e must, in the first place, ascertain fully what are the 

 morphological attributes of all the halolimnic molluscs, so that we 

 can form some idea as to where in the phylogenetic series these 

 particular organisms actually stand; and, secondly, we must get to 

 know definitely the real facts of the distribution of these forms 

 in the African intei-ior. After I had returned from the first 

 Tanganyika expedition we had obtained a certain but an incomplete 

 amount of material with which to ascertain the affinities of the 

 halolimnic forms, but for the second line of investigation we had 

 nothing but the meagre observations to which I have already 



>■ Wliite, C. A. : Proc. U.S. Kat. Mus. AVashingion, 18S2, p. 98. 

 ^ Tauseh, L. : Zeil.sfhr. deutscli. geolog. Gesellseh, Bd. xliv. 1892, p. 697. 



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