108 PKOCEEDINGS OF THE M-1LAC0L0GICA.L SOCIETY 



OEDINAEY MEETING. 



Friday, 11th March, 1898. 



Lieut. -Col. H. II. Godwin-Austen, F.R.S., etc., President, in the Clifiir. 



Mr. J. E. S. Moore opened a discussion upon the Mollusca of Lake 

 Tanganyika, with especial reference to their affinities. 



He pointed out that the problem of the nature, and especially of the 

 origin, of the halolimnic fauna of Lake Tanganyika could be attacked from 

 three distinct directions, or in three methods. These were : — firstly, the 

 evidence afforded by the characters, i.e. the anatomy, of the halolimnic 

 animals themselves, and this he regarded as the most important line of 

 research ; secondly, the results of an investigation into the geographical 

 distribution of these forms ; and thirdly, such evidence as could be 

 gathered by a comparison of the halolimnic shells with fossil forms. 



The results of a morphological examination of the halolimnic animals 

 had made it evident that the fauna of Lake Tanganyika was to be 

 regarded as composed of two series, each entirely distinct in its origin 

 and nature from the other. 



The anatomy of the genera TypJiohia and Bathanalia showed that 

 neither of these forms could be regarded as belonging to the Melaniidse, 

 but must rather be considered as something akin to a Pterocera with 

 a foot that had not been specialized. In the same way the genera Spekia, 

 Tanganyicia, Fai'amelania,, Nassopsis, and Bi/thoceras all distinctly 

 foreshadowed in their anatomy the characters of difiBrent living forms. 

 Thus, Tanganyicia appeai'ed to represent an older type of Littorina, with 

 a Cerithoplanaxoid radala ; Spekia was a Naticoid of a primitive type ; 

 while the Paramelanian group exhibited the nervous system of the 

 Cyclophoridse, but were at the same time indistinguishable concho- 

 logically from the old Jurassic marine genus Picrpiwina. 



It would thus appear from morphological considerations that the 

 halolimnic molluscs were to be regarded as a collection of extremely 

 ancient oceanic forms. 



The distribution of these animals showed that they were exclusively 

 restricted to Tanganyika, and thus confii'med the view that they had 

 nothing in common with the normal fresh-water forms distributed in 

 all the other great African lakes. 



It was thus rendered inconceivable that the halolimnic forms could 

 have arisen under the action of ordinary conditions ; whilst it was 

 equally clear that they could not be regarded as the survivors of an old 

 fresh-water stock : for were either of these suppositions accepted it would 

 be necessary, on account of their distribution, to believe also that the 

 halolimnic group of animals had been destroyed in every African lake 

 but one ; a supposition which might be ingenious, but which, when the 

 number of lakes existing in the African interior was fully realized, 

 became grotesque. 



To the halolimnic animals there thus attached the unique interest that 

 they themselves constituted the few surviving indications of an old sea 

 which once extended into the interior of Africa, and which, judging from 

 the singular identity of many of the halolimnic shells with those occurring 

 in the Jurassic seas, must have been of a fairly ancient date. 



