OF THE THIRD TANGANYIKA EXPEDITION. 93 
the Tanganyika collections, offering a marked contrast to the plankton 
of Nyasa and Victoria Nyanza, in which Stephanodiscus, Cyclotella, and 
Melosira abound. Only one species of Cyclotella was observed in the 
Tanganyika plankton and that very sparingly in the November collections. 
Of the Myxophyceæ, two pelagic species of Nostoc (N. piscinale and 
N. carneum) were present in the August collections, and Anabena Flos-aque 
var. circularis occurred in large quantity from October to February. So 
abundant was this Anabena from October to December that the material 
taken from the tow-nets was of a dull blue-green colour although no spores 
had been developed. A. Tanganyikw, a new, spirally-twisted species, only 
occurred in January and February. Four species of Oscillatoria were 
observed, one of which, O. Zanganyikw, is а remarkable new species nearest 
to the marine O. subuliformis. Another new species of interest is Spirulina 
laxissima. The cells in the genus Spirulina are normally twisted into a rather 
close and narrow spiral, and the laxness of the twists in S. laxissima may be 
due to its adaptation to a pelagic existence. 
The occurrence of Plectonema Wollei, Farlow, in the plankton is also 
worthy of note. This alga is not uncommon in the more aérated of the fresh 
waters of the tropics, but usually occurs attached to stones or large aquatic 
plants, especially in running water. Except for the very rare presence of 
false branches (“ pseudo-rami "), this species appears to me to differ in no 
essential point from Lyngbya majuscula, Harvey, which is the largest of the 
marine species of Lyngbya. May not the presence or absence of false 
ramifieation be due to a difference of environment ? | 
Eight species of the Chroococcacee were observed in the plankton ot 
Tanganyika which were not observed from the other lakes. 
Four species of the Peridiniez occurred, all of which were most abundant 
from September to November, about the time of the maximum period of 
Anabwna Flos-aque var. circularis: Peridinium africanum and Peridiniopsis 
Cunningtonii have been described by Lemmermann as new species. Glenodintum 
Pulvisculus was particularly abundant in one of the November collections. 
From a careful comparison of the collections there are slight indications 
that the plankton of this lake may not be uniform throughout. The con- 
stituents are probably not in the same proportions in different parts of the 
lake at the same time, and further observations will probably show that this 
is especially the case with regard to the relative dominance of Anabena 
Flos-aque and Synedra Acus (together with Nitzschia nyassensis). 
The following table contrasts the relative frequency of the various species 
observed in the collections from July 1904 to February 1905. “сес” = very 
2? [44 5*5 
abundant, “сс” = common, “с” = fairly common, “г” = infrequent, 
“rr” = rare, and “rrr” = very rare. 
