OP THE HALOLIMNIC FAUNA OF LAKE TANGANYIKA. 311 



country occurs towards the top of the Inferior Oolite, where 

 it is extremely local ; and as a proof of the conservative 

 character of some fresh-water organisms, always supposing 

 them to have lived in fresh-water, this form is almost identical 

 with the Paludina vivipara of the present day. I mention 

 this genus as being very characteristic of fresh-water; and on 

 the higher horizon of the Purbeck beds the genus is represented 

 by two other species in great abundance, together wdth many 

 other fresh-water genera. Nevertheless in the Piirbecks, as 

 in the Coal-measures, there are estuarine intercalations when 

 a different set of fossils are found, and in the case of Pahulina 

 langtonensis from the Lower Oolites of Oxfordshire marine 

 gasteropods occur in the same bed. The above statements supply 

 a few facts as to the appearance in time of certain fresh-water 

 organisms ; but the question of their origin seems scarcely to 

 have got beyond the range of conjecture. However, it is in 

 the Coal-measures and in some members of the Jurassic 

 system that the question of the ori.oin of fresh-water molluscs 

 can best be studied at present. The remarkable uniformity 

 in general character of these organisms over very wide spaces 

 is itself a problem as yet by no means solved. 



Before proceeding to study the geology of Equatorial Africa 

 as in any way affording a possible clue to the origin of the 

 halolimnic fauna and especially the gasteropods, which present 

 such a contrast to the average fresh- water molluscs of 

 Tanganyika or of any other African lake, we might consider 

 a possible explanation, which has already been put forward, 

 viz., that some of the halolimnic genera, such as Pciramdania, 

 for instance, might be related to such a stock as Pyrgulifera,* 

 a fossil from fresh-water beds of the Upper Chalk in southern 

 Europe. As far as external appearances go, the halolimnic 

 Paramelania resembles the Cretaceous fresh-water Pyrgidifera 

 quite as much as it does the Jurassic Purimrina, and since 

 Pyrgidifera was nearer in time and moreover a fresh-water shell, 

 it might with more probability be regarded as an ancestral 

 form. Too much stress should not be placed on the resem- 

 blance of a single genus, but it is a fact of some importance 

 that a fresh-water genus of the Cretaceous period is concholo- 

 gically as like the old Purjmrina as any of the Tanganyika 

 shells. 



On the 'jpossihlc transference from marine to fresli-ivater con- 



* Figured on p. 343 of the Tanganyika Problem and referred to on 

 p. 335. 



X 



