1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1143 
exploration mast unfortunately — when taken as a paradigm to a 
sub-arctic lake — be used with great caution. This will best be 
understood when remembering that all animal life in the lake is, 
according to all accounts thereof, only 120 years old, and when 
keeping in mind the extremely volcanic area in which the lake 
is situated. 
The most conspicuous fact with regard to the plankton of 
Myvatn is the non-existence of phytoplankton. Of the Myxo- 
phycern I have only noted some few individuals of the genus 
Anaboena , and this only in a single sample (15th July 1903) and 
at the highest tp. of the water. Not a single plankton Diatom 
is pound, nor any plankton Chlorophycese ; the plankton of Myvatn 
is almost wholly zooplankton. 
While the lake was ice-covered I ordered the samples taken from 
a hole in the ice. In these samples I have found the cosmopolitan 
perennial Eotifer fauna as mentioned above, as well as some bottom 
Crustacea shown in the plankton table. Of course the number of 
animals in these vertical samples, taken in a water column of only 
a few feet, is very small ; yet the samples clearly show that 
in winter all life, even in as cold a lake as this, is by no means 
wholly wanting. Still, I presume that the plankton life in winter 
is greatly reduced, and that it only flourishes during a period of 4 \ 
months, for instance in 1904 from 28th May to 24th October. 
The plankton consists almost solely of Rotifera and Crustacea. 
The Rotifer fauna is probably extremely rich. Among the plants 
contained in the samples I have found many bottom and shore 
Rotifers ; in as shallow a lake as Myvatn, and one which is to such 
a degree covered with vegetation, no limit between a limnetic and 
a littoral region can be said to exist. Besides, we find in the 
samples the entire stock of cosmopolitan perennial Rotifers, but 
none of the periodical summer forms appear. I have only 
found well-marked max. and sexual period for Gonochilus and for 
A-splanchna priodonta, both at the highest tp. of the water (15th 
July). Also with us these Rotifers reach their max. at tp. 12°, 
but as our lakes reach this tp. twice in the year, in May and in 
September-October, they are dicylic with us. 
The Crustacea fauna I also suppose to be very rich. I found in 
the samples many bottom and shore animals, for instance, Macrothrix 
