1904-5.] The Plankton of Thmgvallavatn and Myvatn. 1111 
expect to find , but which are wanting. Of these we may mention : — 
Tabellaria fenestrata, Dinobryon sp., Scenedesmus, Pediastrum, 
Eudorina, etc.,* besides all the Myxophyceee. 
2. Remarks on some Species. 
Chlorophyce^e. 
The Desmids do not play any important part in the plankton, 
but there are some few species which occur in nearly all the 
samples, consequently all the year round. These are some species 
of Staurastrum and one Cosmarium ; the long Closteria, which 
often have been found in plankton, do not seem to exist at all in 
the plankton of Thingvallavatn, nor do we find any species of the 
Xantidia. The max. of the Desmids occurs in October 1902, with 
a secondary max. in June 1903. 
The species observed are rather interesting, as they come near 
or are identical with some of the many plankton Desmids mentioned 
and figured in the papers on phytoplankton from Ireland and 
Scotland by W. West and G. S. West (1902, 1903). As my identi- 
fication of the species may be incorrect, I have drawn figures of 
them (see PI. II. figs. 11-15). 
Cosmarium phaseolus, Breb. — The specimens observed were 
32-38 g long, 30-36 g broad, ab. 18 p thick, and the isthmus 12 p. 
It is with some hesitation that I have referred them to this 
ubiquitous species ; they greatly resemble Mr W est’s drawings of 
C. abbreviatum , Racib., var. on pi. xv. fig. 6 (1903); but the 
semi-cells of the latter are more flattened and more angular, and 
the dimensions are smaller.! 
Staurastrum pelagicum , W. West and G. S. West, l.c. (1902) 
pi. ii. figs. 26-27, p. 46. — This very characteristic species, 
which has been described from Lough Neagh and Lough Beg in 
Ireland, occurs regularly in the samples from Thingvallavatn and 
is the most abundant Desmid in the plankton. As my drawing 
* In a sample from the neighbourhood of Thingvallavatn, a Dinobryon 
occurs ; and also all the other species enumerated as wanting have been found 
in other lakes in Iceland. 
t O. Borge (1897) has given a drawing of a C. phaseolus, 0. achondrum 
(Boldt)? from a lake in the island Mull (Scotland), but it is smaller than 
our form. 
