ANATOMY OF LIMNOCNIDA TANGANYICA, 289 
of the Congo. It appears that the level of the lake has become 
considerably lowered within quite recent times. Such, then, are 
the main physiographical facts relating to the habitat of 
Limnocnida. 
If Limnocnida were the sole representative of a marine 
fauna in Lake Tanganyika, it would be a surprising fact 
enough, but it is associated with other marine forms. Just 
as in the case of the fauna of the Trinidad lagoon, where we 
find the sea-sprung Halmonises accompanied by marine 
Polychete worms, so in the case of the fauna of Lake 
Tanganyika Limnocnida is accompanied by several genera 
of molluscs which are perfectly unique in fresh water, and 
which would most certainly be described as marine forms were 
their true habitat unknown. 
Mr. Edgar Smith (14) especially draws attention to 
Tiphobia horei and Neothauma tanganyicense, both 
of which are quite unlike any genera known from the other 
Central African lakes, and both of which differ from any other 
molluscs of fluviatile or lacustrine origin. 
The question that now arises is, If these organisms are of 
marine origin, how did they get into Lake Tanganyika ? 
The answer is undoubtedly difficult to find. A possible ex- 
planation is afforded us by the analogy of the marine guests of 
the Trinidad lagoon. If geologists, when they know more 
about the geology of Central Africa than they do at present, 
will allow us to imagine a state of things when the level of the 
region of the Great Lakes was more than 2700 feet lower than at 
the present day, and when the Atlantic Ocean extended over 
what is now the plain of the Congo, and Lake Tanganyika 
was a fjord communicating with the ocean, then it would have 
been possible for the ancestors of Limnocnida, of Tiphobia, 
Neothauma, and other molluscs, to have wandered into the 
lake, just as at the present day individuals of the Atlantic 
fauna wander into the Mediterranean. The lake having thus 
received its marine population, was probably gradually separated 
from the sea by the uprising of the land and became an inland 
salt-water basin. At this stage it was possible for one of two 
