1904 - 5 .] The Plankton of Tliingvallavatn and Myvatn. 1129 
most common on 16th and 30th April. As my knowledge of 
the Heliozoa is but slight, and as the animal was badly preserved, 
I sent it to Dr Penard, who kindly wrote me that, according to 
his opinion, it was an Acanthocystis aeuleata, but a somewhat 
smaller specimen than those which he had examined in the Swiss 
lakes, for instance, in Lake Geneva. 
Infusoria.. 
Front onia, Ehrbg. 
In the sample of 16 th April I found a very peculiar body, which 
at first sight caused me a good deal of trouble ; it was a brown sack 
generally c. 200 g long and 50 p broad ; it was quite filled with long 
Melosira chains in numbers of 3-7, and the chains often stuck out 
either at the one end or the other; near the centre I always 
found a black body, and the surface of the creature was covered 
with fine granulations. I had never before in a plankton sample 
seen anything like this creature, and was hardly sure of having an 
animal before me. Dr Penard, who kindly also examined this body, 
told me that it must be an Infusorian, and Dr Roux, to whom 
he showed it, supposed it to be a Front.onia. I wish to offer both 
gentlemen my most cordial thanks for their kind determination of 
the animal. 
It is the first time we have met wdtli an Infusorian of this 
group as a plankton organism ; as far as I remember, I have never 
seen such a one living so exclusively on another organism and, so 
to say, preying upon Melosira and solely upon this single organism. 
Rotifera. 
Polyarthra platyptera, Ehrbg. 
P. platyptera is perennial, although it has not been found in 
a few of the early spring samples. The variety euryptera has 
not been found. P. platyptera has a great max. (14th July) ; 
at that time some individuals with male eggs have been proved ; 
the dark-spined winter eggs are by no means rare, especially in 
the autumn and winter samples. 
