1904-5.] The Plankton of Thingvallavatn and Myvatn. 1119 
From the remarks of Lemmermann, it seems as if the pheno- 
menon is a very common one ; hut if we consult the numerous 
photographs by Wesenberg-Lund (1904), we always will find the 
Melosirce, straight. Dr Wesenberg-Lund also tells me that such is 
the case in the Danish lakes. 
E. Lemmermann very likely is right in taking the movement of 
the water as the cause, but we have herein no explanation of the 
fact that the phenomenon is common in Iceland, hut does not exist 
in Denmark. We want here the experiment, and it cannot he 
very difficult to make it, if one has plankton with Melosirce at his 
disposal. Until that has been done, we must content ourselves by 
affirming the probability of the curvature being an adaptation for 
floating, but we are not able to explain anything of the cause 
which has produced it. 
Rhizosolenia . — In many of the samples I have found the 
peculiar fresh-water Rhizosolenias, but only in the two samples 
from June 1903 did they become rather abundant. The identifi- 
cation of the species is not easy ; at first I thought that all the 
specimens belonged to one species, which must be a form of 
R. eriensis, and that the individuals varied greatly in size and 
shape. But on a closer examination of many specimens, I became 
convinced of the existence of two species or races. I shall here 
give some figures representing the results of measurements of 14 
specimens (in //,). It will be seen that the breadth of the cell 
Table I. 
Indi- 
viduals. 
No. 
Length of 
the Cell. 
Length of the 
Cell without 
Setae. 
Length of the 
Setae. 
Breadth of 
the Cell. 
1 
174 
134 
20 + 20 
8-5 
2 
104 
68 
20 + 16 
7‘5 
3 
110 
74 
18 + 18 
10 
4 
184 
136 
24 + 24 
10 
5 
132 
98 
18 + 16 
8 
6 
160 
122 
20 + 18 
8 
7 
180 
136 
20 + 24 
11 
8 
120 
84 
20 + 16 
4 
9 
120 
90 
16 + 14 
5-5 
10 
160 
120 
22 + 20 
7 
11 
148 
110 
20 + 18 
4 
12 
112 
76 
20 + 16 
45 
13 
100 
70 
16 + 14 
4-5 
14 
134 
94 
20 + 20 
5 
