1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1151 
Ekman (1904, p. 88) states that in the first summer months he 
has occasionally found numerous males, which he supposes to have 
been derived from ephippial eggs. Hot all of the females were 
outgrown, and none of them had eggs. He points out that the 
observations are of interest, because we would then here have the 
only known example, according to which males had been produced 
from ephippial eggs. Till more thorough explorations have been 
carried on, I think it safer to suppose that these males as a first 
brood have been derived from the females hatched from the 
ephippia. 
IY. COMMON RESULTS. 
Lastly, we shall shortly draw attention to the following results 
of the exploration. As compared with the Danish explorations, it 
will readily be seen how much these have been promoted and sup- 
ported by this little investigation. 
1. In Myvatn no phytoplankton has been found at all. 
In Thingvallavatn we have not found any plankton Myxophycese; 
when we add hereto that none of the rather few samples from 
higher latitudes, which have as yet been studied, record Myxo- 
phycese, we may conclude that the Myxophycese do not play any 
conspicuous part in the plankton of more northerly situated lakes. 
When taking into consideration the explorations from Switzerland, 
Germany, and Denmark, we must suppose that the low tp. 
(below c. 12° C.) and the clear water, poor in organic matter, most 
probably form the greatest hindrances to the progress of the 
Myxophycese towards the poles. Eurther explorations may show 
whether the plankton Oscillatoria and Lyngbya are found further 
north than the other Myxophycese ; then their max. may lie at a tp. 
which arctic or sub-arctic lakes, at least for a short time of the 
year, may also arrive at. 
2. The Diatoms constitute the main part of the phytoplankton 
of northern lakes. In Thingvallavatn the Melosiroe and Asterionella 
formosa are the main forms ; they have in temperate regions 
their max. at a tp. of 4°-10°, which is in accordance with their 
occurrence in Iceland. 
On the other hand, Fragilaria crotonensis , whose max. lies 
at tp. 16°, occurs only in a few specimens and at the highest 
