OF THE THIRD TANGANYIKA EXPEDITION. 191 
comparison with the material collected by Dr. Stuhlmann in October 1892. 
The Bacillarieee and Myxophycez are relatively few in number, and the 
phytoplankton is essentially Chlorophyceous, with all the features of a 
Desmid-plankton. 
5. The phytoplankton of Tanganyika is much richer in number of species 
than that of Nyasa or Victoria Nyanza. It contained 85 species, of which 61 
(72 per cent.) occurred in this lake alone. The Chlorophycez were relatively 
few both as regards species and individuals, but the ВасШаме»ж and 
Myxophyceze were both represented by more than the usual proportion of 
species, and occurred at certain times in prodigious quantities. The Proto- 
coccoidee and Bacillariee were especially evident during November, after 
which month there was a general diminution of the Algæ accompanied by 
a great development of Nauplius larvee. Ten species and one variety of 
this phytoplankton were new. 
Several of the species observed in the phytoplankton of this lake are 
usually marine or brackish in habit, and others more nearly approach marine 
species than freshwater ones. Nitzschia Tryblionella, Hantzsch, var. littoralis, 
(Grun.) Van Heurck, Surirella striatula, Turp., and Chodatella | subsalsa, 
Lemm., are marine or brackish-water species. Navicula distincta, sp. n., and 
Surirella Tanganyike, sp. п., more nearly approach marine forms than fresh- 
water ones, and Oscillatoria Tanganyike, sp. n., is much nearer the marine 
Oscillatoria subuliformis, Kütz., than any other described species. Plectonema 
Wollei, Farlow, is another curious Alga in the plankton: certainly a fresh- 
water species, but one which only differs from the marine Lyngbya majuscula, 
Harvey, in the scarce development of false branches—a character which may 
be due to change of environment. The occurrence of no less than three 
species of Nodularia, a genus which is typically of brackish habit, in the 
immediate vicinity of Tanganyika is also a noteworthy fact. 
The Alga-flora of Tanganyika differs very much from that of Nyasa or 
Victoria Nyanza, but its peculiarities are all of them such as could be accounted 
for by the prolonged isolation of the lake. The definite outlet by the River 
Lukuga into the Congo system is relatively recent, having been established 
since the formation of the volcanic cones to the north of Lake Kivu. The 
damming up of the northern end of the Lake Kivu region added a large 
drainage-area to the Tanganyika lake-basin, and marks the close of the period 
of isolation. There is every reason to consider the presence of certain 
brackish-water Algs, and others with marine affinities, as evidence that 
during the period of prolonged isolation the water became increasingly saline. 
Our knowledge of the modifications which some Alge undergo owing to 
change of environment is in support of the view that those Alge of Tanganyika 
exhibiting marine affinities may have been produced by a gradual increase in the 
salinity of the water over an extended period of time. The relatively small 
proportion of Chlorophycee in the plankton, and the large proportion of Bacil- 
