1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1115 
typical S. acus occur, but rarely, in the samples ; the specimens 
observed measured 120-150 g in length, and about 6 g in breadth. 
Gyclotella comta, (Ehbg.) Kiitz. — In some of the samples a 
little Cyclotella has been found (PI. I. figs. 9, 10). Its diameter is 
7-17 /a; it is very difficult to observe any heterogeneousness 
amongst the marginal radiating striae, but perhaps some of them 
are more distinct than the others. Supposing this, and counting 
upon the fact that the frustules are not undulated, I take it as a 
small form of G. comta. The cells always occur separately. I 
have seen a single specimen of which one valve was of ordinary 
shape, the other semi-globular ; this specimen is purely one 
daughter-cell of an auxospore, the semi-globular valve being the 
half part of the auxospore wall. 
The Melosirce play an important part in the plankton of Thing- 
vallavatn. Several species have been found, but only two of these 
are true plankton forms and have a distinct max. in which 
they are the predominant forms. Two species occur more acci- 
dentally ; of these M. varians, Ag., has been observed in most of 
the samples, but it was always rare and often the specimens were 
empty ; thus we are right in taking it as a stranger, its home being 
at the shores. 
Also M. arenaria, Moore, has only been found in a few specimens, 
and without doubt it acts just as the foregoing species ; also of 
this species some of the specimens observed were dead. 
With regard to the other species the matter is different. They 
are true plankton forms, which have been found in all the samples ; 
they are not rare in July 1902, but since then their number 
decreases rapidly, so that they have become very rare in the late 
summer months, when the tp. is above 9 ’5° ; in the autumn they 
begin to grow better, when the tp. sinks below 8°, but it is not 
before it has gone down to 5° that they really begin to develop. 
We now see that they are predominant in the winter, attaining 
their max. in March-April at a tp. of l°-2\ The coarser of 
the two species begins to be numerous in November-February, 
and attains its max. in March-April; while the slenderer grows 
a little less intensely in the beginning and has its max. in 
April-May. 
As it is very difficult to determine the Melosira species, I have 
