1904-5.] The Plankton of Thingvallavatn and Myvain. 1101 
species : Holopedium gibberum, Bythotrephes longimanus, both 
having individuals of an extremely great size ; Daph7iia longispina 
( = hyalina) in different varieties ; Bosmina obtusirostris, Cyclops 
strenuus, and C. scutifer ; the three species of Diaptomus, D. 
laticeps , laciniatus , and denticornis. The following species, which 
form the greatest part of the Crustacea plankton in the lowland 
lakes, Diaptomus graciloides , Leptodora kindtii , Hyalodaphnia 
cucullata, Bosmina coregoni , and Diaphanosoma brachyurum , are 
here almost entirely absent. 
With regard to the propagation of the Cladocera, Ekman has 
confirmed the correctness of my two above-mentioned suppositions, 
and supposes that when Zschokke has arrived at a result differing 
from mine, the reason may be looked for in the fact that the lakes 
explored by him have not had sufficiently arctic conditions. The 
number of eggs which the parthenogenetic females produce is, 
according to Ekman, by no means smaller than in more southern 
countries, it being, on the contrary, often much larger. 
Ekman states, with regard to the seasonal variations, that these 
are by no means as conspicuous as in more southern countries. 
Brehm (1902) has arrived at quite a similar result regarding the 
alpine lake Achen in Tyrol. 
Though I believe that the facts mentioned by Ekman are quite 
correct, I still suppose that his explorations have not made mine 
superfluous. Mine have been carried out in another country and 
have been based on principles quite different from Ekman’s, his 
being an examination of many localities once or only a few times, 
and all in a relatively short time of the year. I, on the contrary, 
have examined but two localities, but these examinations have 
been carried on regularly every fortnight all the year round. My 
explorations are, furthermore, carried on in a country which only 
in a very few places offers life conditions which may be called 
arctic. Still, it might be expected that the results of my explora- 
tions of the Crustacea would be very similar to those arrived 
at by Ekman. 
As I wished a botanist to work out the phytoplankton, I asked 
my friend Mr Ostenfeld, Inspector of the Botanical Museum in 
Copenhagen, to do it. He had recently published a paper on the 
phytoplankton of an Icelandic lake. To my great satisfaction he 
