1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1125 
form under his name ; his drawing is very like mine, and the 
lengths and the breadths of the cells are about the same, but the 
Saxonian R. paludosa has longer setae. Perhaps we should do 
better in making a new species, but at present I do not intend to 
do so. No doubt there are many races of fresh-water Rhizoso- 
lenias connected with each other, so that they form a series, of 
which the extremes are the broad R. eriensis with short setae and 
the narrow R. minima , Levander, 1901, with very long setae. 
There are two ways to go : either take all the races as one species 
or separate them ; whether the one or the other way is correct, 
is at present impossible to decide. The simultaneous occurrence 
of two forms in a sample may be used as an argument against as 
well as in favour of both contentions. We do not know anything 
of auxospores in the fresh-water Rhizosolenias, but if such things 
exist (which is not at all unlikely, as several of the marine Rhizo- 
solenias have been found with auxospores) we therein perhaps 
may find the explanation of their peculiar occurrence in apparently 
distinct but nearly allied races. 
It seems rather peculiar that both species in Thingvallavatn 
occur also in the winter samples, although they have their maxima 
in June. Nearly all the previous records of Rhizosolenias belong 
to the summer, only by Wesenberg-Lund (1904) we find records 
of R. longiseta from every month of the year (p. 68). This circum- 
stance is probably partly due to the fact that most of the plankton 
investigations have been made in the summer, and partly on 
account of the very thin and small Rhizosolenias having been over- 
looked when they were not present in large quantities. 
The fresh-water Rhizosolenias have a very wide distribution ; the 
most common form is R. longiseta , Zach. Of R. eriensis we have 
records from Canada (Lake Erie), Finland (two lakes), Germany 
(two lakes), Sweden (one lake), Scotland (two lakes), Switzerland 
(two lakes), and Italy. Worthy of notice is the very scattered 
occurrence. Concerning R. paludosa, Zach., the records are scarcer ; 
besides Germany, we have more or less reliable statements 
from Scotland (Loch Shin), Denmark (Soro lake), and Finland 
(Yalijarvi). At last we must remember the most delicate form, 
R. minima , Levander, found in the bay of Wiborg (Finland), 
together with Altheya and other fresh-water forms. 
