86 MR. G. 5. WEST ON THE FRESHWATER АТСА 
of Melosira granulata and of M. nyassensis in the African plankton were 
either quite straight or almost straight, and the same remarks apply to the 
species of Melosira in the Scottish and Irish plankton. Ostenfeld points out 
that the Melosira-filaments are straight in the Danish plankton *, but 
commonly coiled in plankton he examined from a lake in Iceland. 
I am notaware of a definite spiral character, such as the one just mentioned, 
having been developed by any Algs other than limnetic species f, and 
another equally well-marked limnetic feature is exhibited by certain of the 
smaller species of Synedra, in which there is a tendency to assume a colonial 
character owing to the grouping of the frustules in radiating clusters 1]. 
Special eharaeters of this nature, which appear only to be fully aequired 
in the plankton, are most probably directly concerned in increasing the 
floating-capacity of the species; and the rearrangement of the mucus en- 
veloping all these plants to a greater or less degree, which must be caused by 
the acquirement of such peculiarities, may also assist their capabilities of 
suspension. 
A strict comparison between the phytoplankton of the three lakes is not 
possible from the present material, owing to the fact that the collections from 
each lake were not made at the same seasons of the year. Nevertheless, 
considerable differences exist which are in no way due to seasonal variations 
of the plankton-organisms, and these differences are of a highly interesting 
character. A more complete comparison can be made between Tanganyika 
and Lake Nyasa than between either of these lakes and Victoria Nyauza, ав 
collections from the two former lakes extend over a corresponding period of 
about eight months. 
The table immediately following has been constructed to show all the 
species which have been observed in Dr. Cunnington's plankton-collections ; 
after which are paragraphs relating to the peculiarities of the plankton of 
each lake, Tanganyika receiving rather fuller treatment than the others. 
* Cf. also C. Wesenberg-Lund, ‘Studier over de danske Süers Plankton, Kjóbenhavn, 
1904. 
T In the genera Spirulina and Arthrospira the filaments are usually very much longer and 
the spirals of much less diameter than in the limnetic spirally-twisted forms. Spirulina 
laxissima, a new species from the plankton of Tanganyika, is particularly interesting, as in 
correlation with its limnetic existence the filaments are short and the spiral twists of much 
greater diameter than usual. 
{ Lemmermann has created а subgenus “ Belonastrum” for the plankton-species which 
form these stellate or radiating clusters. Cf. Ber. Deutsch. Bot. Ges. xviii. (1900) p. 30. 
In this subgenus of Synedra Lemmermann places four species (S. actinastroides, S. berolinensis, 
S. revaliensis, and 5. limnetica) and several varieties, of which he gives a summary in the 
same Journal, xxiv. (1906) pp. 537, 538. 
