094 Bulletin American Museum of Natural History [Vol. LIT] 
that.the anatomy of the Tanganyikan thalassoid mollusks, as known at 
present, shows that they are related to the living fresh-water melanians. 
Moreover, Moore’s hypothesis is contradicted by the geological structure 
of the region of Lake Tanganyika.' There are no indications that the 
sea extended anywhere near the lake during the Mesozoic, as no’ marine 
deposits of that period are known in the interior of Central Africa, the 
rocks all belonging to continental or lacustrine formations. This region 
appears to have undergone no marine transgression since the middle of 
Paleozoic times. It is now generally admitted that after the close of the 
Paleozoic the earth crust of this part of Africa was dislocated by a series 
of tremendous fracture movements, so-called faults. These movements 
covered a long lapse of time and must have been interrupted by several 
periods of relative quietness. In the region of Tanganyika they probably 
began about the middle of the Mesozoic and continued till the present 
day: even now earthquakes dre quite frequent on the shores. The lake 
nowadays fills the bottom of the southernmost and deepest portion of 
a huge trough, known as the Albertine Rift, which resulted from the suc- 
cessive faulting movements. It is therefore in no way a residual sea. 
On the whole, the most plausible explanation of the peculiarities of 
the Tanganyikan mollusk fauna seems to be that it is a mixture of several 
distinct faunz, all derived from FLUVIATILE ancestors, but which have 
reached the lake at different epochs of its geologic past. So many have 
been the vicissitudes of the history of the lake and of that of the sur- 
rounding territory, that connections must have been repeatedly estab- 
lished and severed again with other hydrographic basins. It seems 
equally safe to assume that at intervals Tanganyika formed for some 
length of time a closed basin, where truly marine-like conditions pre- 
vailed, perhaps even more pronounced than nowadays, as for instance in 
a higher salinity. Thus there was ample opportunity for repeated immi- 
grations of fluviatile stocks from neighboring river basins, while at least 
some of them could later’ evolve into peculiar types better adapted to 
their new environment. It is quite possible that some of the most strik- 
ingly thalassoid genera are the direct descendants of fluviatile forms which 
migrated into the lake at the beginning of its history, perhaps during the 
Mesozoic. But, even in their case, it is likely that they have been deeply 
1See for a discussion of the geological side of the argument: 
Cornet, J. 1896. ‘Le Tanganyika est-il un Relikten-See?’ Le Mouvement Géographique, Nos. 
' 25 and 26. 
Soi EK. 1901. ‘Ist der Tanganyika ein Relikten-See?’ Petermann’s Mittheil., XLVII, 
pp. 275-278. 
Hudleston, W. H. 1904. ‘On the origin of the marine (halolimnic) fauna of Lake T ika.’ 
Geological Mag., pp. 337-3882, 2 Pls. ee 
