342 
THE TANGANYIKA PROBLEM . 
continents, arose from a sea which, like the Atlantic, 
originally occupied a position somewhere between the two. 
The fact that the consideration of the distribution of the 
fish-fauna of Tanganyika points to this fauna having origi- 
nated in a sea lying to the west of that lake, is extremely 
interesting ; for we saw, when dealing with the geology of 
these districts, that the distribution and character of the 
aqueous deposits of the region of Tanganyika point to the 
west as the only direction in which there seems any like- 
lihood of there having been a former extension of the lake, 
or a junction between it and a sea covering a part, or the 
whole, of the unique Congo basin. 
A similar indication is afforded when we study the life 
now found in the Congo. Thus, it is known, that one 
of the remarkable sponge of Tanganyika, Potamolepis , 
inhabits the lower reaches of the river. A certain number 
of fishes are common to both waters, while the mollusc 
Paramelania of Tanganyika is extremely similar in its 
anatomy to the genus Tympanotamus, occurring low down 
in the Congo, in its fresh-water, and also in the sea on 
the west coast. 
We have also the fact that far up the Congo in its fresh- 
water there occur red Algae, corresponding to the red 
sea-weeds of the ocean. But by far the most interesting 
fact yet brought to light about the Congo fauna, has resulted 
from Mr. Boulenger’s investigation of the fishes which 
were collected in different parts of the river by the officials 
of the Congo -Free State. 
It was shown, as a result of this investigation, that the 
Congo contains fifty-one species of the typical African 
family, the Mormyridae. The Congo, in fact, repeating 
with respect to this group the same peculiarities which 
Tanganyika exhibits with respect to the Cichlidae. By far 
