174 J. B. S. MOORE. 



views, and there is direct evidence to show that Nyassa has been 

 a fresh-water lake longer than Tanganyika. On the shores of 

 Nyassa there are old raised beaches, forming white limestone 

 cliflFs which contain the fossilised remains of the shells now 

 living in the lake. But in these old lake beds there are no 

 traces of any Halolimnic forms, and this is all the more con- 

 clusive as the shells of the Halolimnic molluscs are much more 

 solid and durable than those of the fossilised fresh-water 

 forms. 



The second hypothesis, that which suggests that the Halo- 

 limnic fauna may be the surviving representative of an ancient 

 fresh-water stock which has become extinct, has great attrac- 

 tions, as it conforms to a famous geological speculation, 

 and has at first sight the appearance of a certain modicum 

 of positive support. For the shells of the Paramelanias of 

 Tanganyika have been independently supposed by White and 

 by Tausch to be identical with the extinct estuarine or 

 brackish Pyrguliferas of cretaceous Europe, America, or 

 Africa.^ 



The type of shells possessed by these forms has been, 

 however, repeated so often by so many widely separated 

 molluscan types, such as in the Melanias, Litorinas, Pur- 



' On further examination it appears— (1) That Ihe genus Paramelania 

 of Tanganyika is similar to the cretaceous Pyrgulifera; (2) but that the 

 genus Pyrgulifera, so far as some of its representatives go, is concho- 

 logically indistinguishable from the old marine Jurassic genus Purpuriua, 

 and that the Nanopsis of Tanganyika corresponds to one section of this 

 genus, the Paramelania to the other. (See Hudleston's figs., Plates i 

 and ii, and text p. 85 — 95, ' Jurassic Gasteropoda,' Palseontographical Society, 

 vol. xli, 1887.) It would thus appear that the marine genus Purpurina 

 became a fresh-water form, as so often happens in Cretaceous times. We 

 find, however, that other Halolimnic Gasteropods, Bathanalia, the so- 

 called Lithoglyphus, and Limnotrochus, are also indistinguishable 

 from marine Jurassic forms, which are not found in any Cretaceous forma- 

 tion, fresh-water or marine. Consequently the geological evidence on this 

 matter distinctly favours the old marine origin of the Halolimnic fauna ; but 

 it places their original marine existence much further back than I had even 

 dared to suggest. I shall discuss this most interesting line of investigation 

 fully in a special memoir. 



