1904-5.] The Plankton of Thingvallavatn and Myvatn. 1127 
but it seems most natural to place it in the genus Gymnodinium. 
Together with these Gymnodinium-like cells a few specimens of a 
true Peridinium have been found, and these become more frequent 
in the samples from April, when the Gymnodinium-like cells 
decrease in number. From the similarity of the contents in the 
Peridinium (PI. I. fig. 18) and of the naked cells, as well as from 
a few specimens of the Peridinium in which the cell-contents are 
breaking out (PI. I. fig. 19), it will be evident that we have two 
stages of one and the same organism. The cell-wall is rather 
thin, and it is difficult to discern the plates, but after treatment 
with iodine-zinc-chloride the wall becomes reddish-violet and the 
sutures to a certain degree visible. The arrangement of the plates 
(PI. II. fig. 18) of the apical limb is about the same as in P. 
umbonatum, Stein, but I have been unable to see the plates of 
the antapical one, so the placing of our organism in the genus 
Peridinium consequently is based on analogy. Around the 
antapical part of the longitudinal furrow three small spines occur ; 
they are merely prominent membranous prolongations of the 
sutures (PL II. fig. 18); the middle one is the most prominent; 
it is linear when seen in a ventral view, but triangular when 
seen in more dorsal view. These three spines distinguish our 
species from the P. umbonatum , Stein, to which it is nearly allied. 
In the literature I have found a short description of a Peridinium 
aciculiferum named by E. Lemmermann (1900, p. 28), which 
agrees very well with our species, but as I did not dare to run the 
risk of identifying the two forms merely on the basis of but 
such a short description without any drawing, I asked Mr 
Lemmermann for a drawing or a sample containing his species. 
He has been so kind as to send me a drawing as well as a 
sample, for which I am much indebted to him, and after examina- 
tion of the latter I do not hesitate to take my form as identical 
with his species, and consequently I name it P aciculiferum , 
Lemm. Perhaps the spines in his form are a little longer, and 
more prominent. He writes that his species, which has been 
found in a lake in the neighbourhood of Berlin, has numerous, 
discoid, brown chromatophores ; the above-mentioned refractive 
bodies are perhaps chromatophores, but it is impossible to decide 
on the question when having only preserved material at hand. 
