1112 Proceedings of Royal Society of Edinburgh [sess. 
will show, the Icelandic form is quite like the Irish ; the dimen- 
sions of the cell are also about the same (long. 40 g, lat. cum 
spin. 67 g, sine spin. 46). W. West and G. S. West have later 
(1903) described a S. pseudopelagicum only differing from S. 
pelagicum by its hollow processes and a somewhat different 
shape of the cells ; but the Icelandic form is more like the 
true S. 'pelagicum. The two species have in common the two 
large spines at the apices of the processes, hut in a specimen 
from Thingvallavatn I have observed three spines at one of the 
processes. 
S. paradoxum , Meyen. — After S. pelagicum the most common 
plankton Desmid in Thingvallavatn is a form of the variable 
S. paradoxum. It is always quadrangular (I have not seen a 
single triangular specimen in the samples), and resembles some- 
what the form figured by W. West and G. S. West (1903) 
pi. xviii. fig. 4, hut is slenderer. Long, sine proc. ah. 40 g , 
lat. sine proc. ah. 22-24 g, lat. cum proc. ah. 100 g. It differs 
much from the commonest plankton form of S. paradoxum , viz., 
var. longipes, Nordst. 
S. breoispinum, Breb. — The form which I have placed under 
this name on account of its resemblance to Buffs’ figure of it 
is also a constant form in the samples. My drawings show 
the shape of it. Long. ab. 50 g, lat. sine spin. ab. 55 g , lat. cum 
spin. ah. 65 g, lat. isthmi ab. 12 g. 
S. Bieneanum, Babenh., forma. — Although this species does 
not occur as constantly in the plankton of Thingvallavatn as the 
above-mentioned ones, it has been found in so many of the samples 
that I think it is a planktonic species. I am not convinced of 
the identification of this species. It is larger than the ordinal’)' 
S. Bieneanum , being 60 g long and 52 g broad, hut Mr Borgesen, 
who has seen my drawing, agrees with me in counting the 
Thingvallavatn form as a variety of S. Bieneanum nearest to the 
var. ellipticum of Wille (1879, pi. xiii. fig. 49); the semi-cells 
are alternating. 
Besides these five species, several other Desmids have been 
observed in the samples ; but as they occur without any regularity, 
and only in one or a few specimens, I have reckoned them as 
strangers to the plankton and therefore omitted them in the list ; 
