1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1157 
occurring at midsummer time at the same tp. (12° C.) as in 
the southern ones. 
8. Ekman (1904, p. 68) has, with regard to its Crustacea, ad- 
joined Iceland as an appendix to the sub-region Greenland, which 
is a part of his boreo-glacial region. The reason for so doing he 
declares principally to he the common occurrence of Diaptomus 
minutus , hitherto not found outside of North America and 
Greenland. 
Nothing in the present little exploration is in contradiction to 
Ekman’s point of view 7 . All plankton Crustacea hitherto found 
belong to the common boreo-subglacial society pointed out by 
Ekman. Owing to incomplete explorations of the island we have 
missed different species, especially Bythotrephes. Further ex- 
plorations will undoubtedly also bring to light more species of 
Diaptomus and Cyclops. It must be hoped that the great 
collections from c. 60 localities made by Mr Soemundsson may 
find their adapter. It must be noted that, in Iceland as well 
as in almost all other parts of the boreo-subglacial region, all the 
plankton Crustacea, which are characteristic for the limnetic region 
of the more southern lowland lakes (Ekman), are missing. 
9. With regard to the life cycle of the Daphnids, I refer to 
p. 1149. The explorations have confirmed the fact that the other- 
wise polycyclical Cladocera will be monocyclical nearer the 
poles, the autumn sexual period being precluded ; in more northern 
latitudes the parthenogenetic propagation is of inferior significance 
in comparison to the sexual one. 
10. From Ekman’s explorations in the Sarek, James Murray’s 
and my own in Scotland, and the present report from Iceland, added 
to the numerous explorations in Central European lowland lakes, 
we may presume that the great seasonal variations of the plankton 
organisms are restricted to the lowland lakes with their high 
summer tp. They are wanting in all those lakes whose summer 
tp. never rises above 12°, the very tp. at which the variations 
begin in the lakes of the Central European plain. 
As a general result of the exploration, we wish to draw attention 
to the following conclusions: We think it probable that the 
plankton of the arctic lakes, to a much greater degree than in 
more southern countries, mainly consists of zooplankton, and that 
