OF THE THIRD TANGANYIRA EXPEDITION. 85 
with that of the British lakes with regard to Из Desmid-flora. Moreover, 
the most abundant Desmid is Staurastrum limneticum, Schmidle, a species of 
a similar type to S. Ophiura, Lund., which latter is generally distributed in 
many lakes of the western areas of the British Islands. 
The Surirellacee are well represented, and certain handsome species of 
Surirella seem to have firmly established themselves in the plankton. This 
is again strictly comparable to the establishment as plankton-species of 
several large Surirellas in the Scottish and Irish lake-plankton *. Cymato- 
pleura Solea seems also in many cases to have become a plankton Diatom, 
and in the plankton of Victoria Nyanza there is a large species which I have 
named Cymatopleura Nyanse. 
Species of Celospherium, which constitute no small amount of the European 
lake-plankton at certain seasons of the year, appear to be quite absent from 
these African lakes. 
As in the European plankton, there is a tendency among certain of the 
Alge to a spiral growth, which results in the production of close-coiled 
filaments. In the Scottish and Irish plankton this feature was principally 
confined to the filaments of Anabena Flos-aque and A. circinalis, to certain of 
the small species of Mougeotia, and to the disposition of the frustules in the 
colonies of Asterionella and Tabellaria. In the African plankton a spiral 
twisting is a conspicuous feature of the Anabena-filaments, but the spirals 
are of a more regular type and the filaments relatively much shorter. In 
Lyngbya cireumcretum the same tendency to spiral growth has resulted in the 
formation of short spiral filaments which have the appearance of little coils of 
wire. The same spiral character is exhibited by Lyngbya contorta, Lemm. 
Ostenfeld + has described and figured from a lake in Iceland, and Volk 1 has 
mentioned as occurring in the Elbe, a similar coiled condition of the filaments 
of Melosira granulata. 
It has already been suggested $ that the development of this coiled 
condition is a limnetie character, and it is certainly of great interest to find 
this character in so many different species апа genera of the phytoplankton. 
Lemmermann || attributes the curvature of the Melosira-filaments to the 
movements of the water. This may be so, but how is it that the majority of 
Melosira-filaments met with in the plankton are quite straight? The filaments 
* W. & G. S, West, in Trans. Roy. Soc. Edinburgh, xli. part З (1906), р. 515, t. 1. 
figs. 2-4, t. 2. figs. 4 & 6. W. & G. S. West, in Trans. Roy. Irish Acad, xxxiii. sect. B (1906), 
р. 89, t. 6. figs. 1 & 3, t. 8. figs. 2 & 3, t. 9. fig. 6. 
+ C. Н. Ostenfeld, in Botanisk Tidsskrift, xxvi. (1904) p. 233, fig. 5 (p. 232). 
t В. Volk, Hamburgische Elb-Untersuchung I., Hamburg, 1903, p. 113. 
$ №. & 0. S. West, in Journ. Linn. Soc., Bot. xxxv. (1903) p. 524 (Mougeotia sp.) 
Ostenfeld, 1904, Z. с. p. 233 (Melosira granulata f. curvata, Grun.); W. & G. S. West, in 
Trans. Roy. Soc. Edinburgh, xli. (1905) p. 497 (Mougeotia sp.). 
| E. Lemmermann, in Ber. Deutsch. Bot. Ges. xxii. (1904) p. 17. 
