1152 Proceedings of Royal Society of Edinburgh. [sess. 
tp., and consequently does not play any important part in the 
plankton. The occurrence of two Rhizosolenice in nearly all the 
months of the year, although only fairly common in June (tp. 
7°-8-5°), is very interesting and indicates probably, as several 
other facts do, that Thingvallavatn by no means may be regarded 
as an arctic lake. It is only a northern lake in want of the 
higher summer tp. ; it differs considerably from a true arctic lake 
in only being ice-covered for a short time, and even this does not 
occur every year. The same difference must also be remembered 
when comparing it to alpine lakes. 
Corresponding hereto we do not find in Thingvallavatn the 
large quantities of Cyclotella which occur, e.g., in Switzerland. 
If we compare our results concerning Diatoms with the few 
statements from arctic lakes, we find that in Greenland, at c. 71° lat. 
N. (Vanhoffen, 1897), in Bear Isle (Lagerheim, 1900), and in Lule 
Lappmark (A. Cleve, 1899), the plankton Diatoms do not play any 
prominent part in the plankton, the phytoplankton being very poor. 
The fact is worth noticing, that these statements are based on 
samples collected in very small lakes or, more correctly, in ponds 
(except those from Lule Lappmark). 
We think that the large quantities of Melosira and Asterionella 
in Thingvallavatn show how nearly allied its plankton is to that 
of north-western Europe. Besides the above-mentioned forms, 
several other Diatoms occur regularly in the plankton of Thing- 
vallavatn, viz., Fragilaria construens , F. capucina , Synedra acus , 
and S. ulna. These forms are not of as typical a limnetic character 
as the others ; we presume that their home is the shores of the 
lake, from whence they are driven into the open lake by winds 
and waves. On account of the low tp. they are able to float 
for rather a long time. The papers by Lagerheim (1900) and 
Ostenfeld (1904) mention such tycholimnetic forms, which with 
higher tp. probably quickly would sink to the bottom. 
3. As might be expected, the limnetic Chlorophycese are not 
abundant in Thingvallavatn ; the main form is Sphcerocystis , the 
same which predominates in alpine lakes and which is probably 
very common all over the Central European plain. In Thingvalla- 
vatn its max. occurs at 7° C. ; with regard to its max. in other 
lakes we have no exact information. The rather common 
