1098 Proceedings of Royal Society of Edinburgh. [sess. 
a small lake in Iceland. On the other hand, the occurrence of 
Geratium , as well as of Volvox and Eudorina in Greenland and 
Iceland, show how cautious we must he in drawing our conclusions. 
The tp. at which a species has its max. and occurs in large 
quantities in the lakes in the temperate regions and in the lowland, 
will not always be a necessary condition for the progress of the 
species towards the North. Consequently it is always rather 
hazardous to draw conclusions from its max. tp. in temperate 
regions as to the northern limits of its geographical distribution. 
Of the other genera of Peridinese we probably will meet species of 
Peridinium and Gymnodinium ; the P. willei seems, according to 
papers of Huitfeldt Kaas (1900), Levander (1901), Ostenfeld (1903, 
1904), West (1903), and Lemmermann (1904 a), to he a species 
with northerly (and north-westerly) distribution, hitherto found in 
Norway, Finland, Iceland, and the Faeroes, Scotland and Sweden. 
Among the other Flagellata the genus Dinobryon will no doubt he 
common. We are aware of its occurrence in Greenland, Iceland, 
the Faeroes, the Lappmark and the Murman coast, as well as 
further south. It will, together with the Diatoms, be the 
prominent form in the phytoplankton of lakes in higher latitudes. 
One species has been found in samples from Iceland (Ostenfeld, 
1904), two species predominate in Greenland (Vanhoffen), and 
one in Lule Lappmark (A. Cleve). The Eotifera will only be 
represented by the cosmopolitic perennial dicyclic group, the 
monocyclic periodical summer forms most probably being wholly 
absent. It appears that there is a difference between this 
plankton Crustacea and ours. The life cycle is, as mentioned 
above, much simpler, and the seasonal variations are inconspicuous. 
As I wished to have my results and suppositions as exact as 
possible before writing the second part of my plankton paper, 
there was only one thing to be done : to contrive regular fort- 
nightly plankton explorations in lakes from higher latitudes. I 
wished to effect such explorations in pronounced arctic lakes as well 
as in lakes with a low summer tp. (never more than 12° C.). I 
consequently endeavoured to procure regularly collected samples 
from Greenland as well as from Iceland. AVith regard to Green- 
land I was rather unfortunate. I tried in different ways to bring 
about explorations ; in the time from November 1902 to February 
