1934 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1139 
broods; the sexual period does not begin till the last days of 
September, and continues till January. 
The number of eggs in every egg-sack at 30th September is 
about 4, in October-November 6-7, but in December- January it 
diminishes again (2-3). 
I suppose that further explorations will show that the limnetic 
region of the Thingvallavatn contains several other plankton 
Crustacea .than those mentioned in this paper. It must be kept 
in mind that all the samples are surface samples, and I consider 
it most probable that different species, especially Bythotrephes , 
may be found in deeper waters. Richard and De Guerne men- 
tion, as stated above, different bottom and shore species, among 
which we also find Bosmina arctica (B. obtusirostris). This species 
may be considered a plankton as well as a shore organism. I only 
wish to emphasise that none of my numerous plankton samples 
ever contained a single B. arctica. 
III. MYVATN. By C. Wesenberg-Lund. 
1. General Remarks. 
Myvatn is situated in the northern part of Iceland, in latitude 
65° 33' N., 292 m. above sea-level, and is'nearly 27 square kiloms. 
in extent. The lake has been formed in down-sunken parts 
of enormous lava torrents. The bottom consists almost entirely of 
lava, and is nearly everywhere surrounded by widely extending 
lava grounds. Along the shores, and forming islets in the lake 
itself, the lava is congealed in very peculiar and fantastic columns. 
The lake is situated in a volcanic area, which even now-a-days may 
be considered extremely active. Upon the east side of the lake 
very many solfatares occur. According to Thoroddsen, the ground 
is here actually seething with hot vapours, and it is dangerous to 
walk upon it; little hillocks of sulphur are very common and 
alternate with pools of mud, which incessantly boil and bubble, 
while ejecting bluish-black clay mud. 
The surrounding country, especially the northern and eastern 
part, is extremely void of water, as all the rain is absorbed by the 
porous volcanic soil. It seems as if the water, partly through 
