192 MR. С. S. WEST ON THE FRESHWATER ALG! 
lariee and Myxophycew, is also an indication that the water of the lake was at 
one time much more saline than it is at present. 
Collections made within a few days of one another from different parts of 
Tanganyika differ so much, even in their dominant constituents, that it is not 
at all improbable that some of these plankton-organisms may occur in large 
shoals of more or less limited extent. From available evidence it is not 
possible, however, to make a definite statement on this point, as collections 
were not actually made in different parts of the lake at the same time; but 
the dominance of Anabena Flos-aque var. circularis in some collections, and 
of Nitzschia nyassensis and Synedra Acus in others obtained very shortly 
afterwards from other parts, indicates that the plankton of the lake is by no 
means of a uniform character. The Nyasa plankton collections also furnish 
evidence of a similar absence of uniformity, but, owing to their less compre- 
hensive nature, to a much less marked degree. 
In large bodies of fresh water, such as these Central African lakes, it would 
appear that a single sample of plankton obtained in a stated locality must not 
be regarded as representative of the plankton of the entire lake. 
EXPLANATION OF THE PLATES. 
PLATE 2. 
Photomicrographs of plankton from Lake Nyasa. 
Figs. 1-16 x 100; figs. 17-20 x 150. 
Figs. 1 & 2. Varieties of Pediastrum simplex; 3. Stephanodiscus Astrea; 4. Melosira 
granulata, var. angustissima ; D. Synedra Acus; 6 & 7. Melosira nyassensis ; 
8. Anabena Flos-aque, forma; 9. Surirella bifrons, var. tumida. 
10. Closteriopsis longissima ; 1l & 12. Surirella Nyasse, with numerous attached 
Vorticelle or similar Infusoria. 
18 & 14. Melosira granulata, var. angustissima ; 15 & 16. Anabena Flos-aque, forma. 
17 & 18. Pediastrum simpler, var. clathratum; 19. Melosira granulata, var. angus- 
tissima; 20. Anabena Flos-aque, forma. 
PLATE 3. 
Photomicrographs of plankton from Victoria Nyanza. 
All x 100. 
Figs. 1. Staurastrum cuspidatum; 2. Surirella Malombe; З & 4. Surirella Fiillebornii, var. 
elliptica; 5. Melosira nyassensis; 6 & 7. Masses of partially decomposed organic 
matter which form a feature of the plankton examined from this lake; 7 is 
probably the excreta of some animal. 
8-10. Staurastrum limneticum; 11. S. leptocladum, forma africana; 12. Sphincto- 
siphon polymorphus, young colony ; 13 & 14. Surirella Malombe ; 15, Staurastrum 
tohopekaligense. 
16. Pediastrum simpler, var. clathratum; 17. Surirella, sp. ? ; 18. Lyngbya circum- 
cretum; 19. Floating and partly decomposed organic matter. 
