1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1093 
respectively, which had been gathered during journeys with quite 
other objects in view. 
Explorations on a very large scale were started years ago by Mr 
Huitfeldt Kaas in the Norwegian lakes; but to my knowledge 
only a minor part has as yet been published. 
When comparing the results of my own explorations with those 
in foreign countries, I arrive at some general conclusions. These, 
I suppose, may be considered as facts of great probability. I have 
taken the liberty to give an account of some of the points which 
may be supposed to be of the greatest interest, in the following pages. 
(Owing to the frequent use of the words “temperature” and 
“maximum development,” I shall in the following abbreviate 
these to “tp.” and “max.” respectively.) 
1. Myxopliycece . — The bulk of these belong to the pond-like 
lakes with high summer tp. and rich in organic matter ; most of 
them reach their max. only at a tp. of 20° C., and the only 
plankton Myxophycese 'which play a somewhat conspicuous part 
in the cold, clear alpine lakes are Oscillatoria rubescens (max. at 
tp. 5°-10° C.) and Anabcena flos aqua} (max. 16°-18° C.). 
2. All the European fresh- water Diatoms seem mainly to 
attain their max. at a tp. below 15°-16° C. Only the max. of 
Fragilaria crotonensis is reached at a tp. of 13°-16° C. The 
greater part of the remainder reach their max. at a much lower 
tp. (7°-10° C.). 
3. Chlorophycece . — Nearly all of these are pond forms and 
only very few belong to the pelagic region of the larger lakes. As 
such may be mentioned Spheerocystis schroeteri. Dictyosphcerium 
pulchellum and a few species of Oocystis, Botryococcus braunii , 
Raphidium braunii , etc. Only very few of these species are of 
any importance in the pelagic regions, and the rest may all be 
regarded as tycholimnetic. 
4. Peridinece . — The only common plankton-organism within 
this group to be found in the European lakes is Ceratium 
hirundinella. It is a summer form, with max. occurring at the 
highest tp. of the water. Different species, especially of the 
genera Peridinium and Glenodinium , appear in the pelagic 
region, but they generally appear in small quantities and have 
been studied but little. 
