454 Mr. J. E. S. Moore. Zoological Evidence for the 



these facts disclose is also true and characteristic of all the other 

 halolimnic animals I have named. It is thus rendered evident that 

 the entire halolinmic fauna as it exists in Tanganyika now is some- 

 thing completely distinct from and superadded to the normal African 

 lake fauna as a whole. This fact is of the utmost import when we 

 attempt to ascertain from what source the halolimnic animals have 

 sprnng. 



The isolation of these animals shows conclusively that they cannot 

 have arisen, so to speak, de novo in Tanganyika through the effect of 

 the conditions under which they live, for if this were so there would 

 have arisen similar halolimnic animals under the apparently similar 

 conditions which exist elsewhere. For the same reason they cannot 

 he regarded as the surviving representatives of an older fresh-water 

 stock, since were this the case we should have to believe that this 

 old stock had been destroyed in every African lake but one. Nyassa, 

 moreover, appeal's to have been a fresh-water lake longer than Tan- 

 ganyika, yet in the Post- pleistocene deposits which occur along its 

 shores no halolimnic fossils have been found. 



Now it is perhaps conceivable that prawns, which are active 

 vigorous organisms, could by great exertions have made their way up 

 the numerous falls along the single effluent of Tanganyika from the 

 sea in recent times, for they have certainly thus entered many 

 lakes already known. But with respect to the remaining halolimnic 

 organisms, there is a singular feature common to them all which 

 effectually precludes any possibility of this. All these animals are 

 incapable of being directly associated with any living oceanic species. 

 This fact alone demonstrates conclusively that the halolimnic fauna, 

 wherever it came from, must be old. It has either had time to 

 modify into its present condition from forms which a.re already known, 

 or, what is more probable, it has more or less adhered to the characters 

 of the older types from which it sprang. Delicate organisms, such 

 as the Medusa, could not have found their way up the effluent as it 

 now exists: it is barely conceivable that they can have been carried 

 overland, while it is altogether out of the question to suppose that 

 either of these processes could account for the presence of the halo- 

 limnic molluscs in the lake, as these are almost exclusively deep water 

 forms. The genus Typhobia and the genus Bathanalia are generally 

 beyond the hundred fathom line. Limnotrochus and Symolopsis are 

 never found in less than 200 feet, and they occur up to 700 feet. 

 The morphology of these molluscs is therefore of the first importance 

 in determining the nature of the halolimnic group, for if the affinities 

 of these organisms have been misinterpreted, and if in reality it can 

 be shown that they have been derived from ancient oceanic types, 

 they must have made their way into Tanganyika from the sea under 

 widely different conditions from those which now exist ; in fact, the 



