] 904-5.] The Plankton of Thingvallavatn and Myvatn. 1123 
Table Y. 
Breadth of the Gell (/a), 250 dried Specimens. 
4 6 
5 16 
6 24 
7 26 
8 . . 43 
9 13 
10 21 
11 23 
12 43 
13 18 
14 10 
15 5 
16. . . 2 
The shape of this curve is exactly like that of Table IV., the 
same two climaxes of 8 jul and of 12 /x exist here, also the same 
extremes of 4 /a and of 16 p. 
As mentioned above, the broad race bears resting spores , while I 
have not been able to discover any spore-bearing specimen of the 
narrow race. The spores occur only in the samples from June 
1903. My drawings (PI. II. figs. 2, 3) will give the shape of such 
a resting spore seen from the broad as well as from the narrow 
side ; they have one convex and one concave valve, and are much 
more silicious than the other part of the cell ; the whole plasmatic 
content of the cell is concentrated in the spore, in which the 
nucleus and the chromatophore substance is easily shown. The 
spore-bearing specimens vary in breadth from 9 to 16 fx (in dried 
condition), 11 to 13 g being the ordinary figures. 
If we wish to consult papers dealing with the fresh-water Rhizo- 
solenias, we will find many notices scattered in different publications. 
The first-described species is R. eriensis , H. L. Smith, found in the 
large North- American lakes and later also registered from Europe 
(Italy, Switzerland, Germany, Finland, Sweden, and Scotland), 
but always only in a single or in few lakes in each country. To 
this species, of which two varieties have been proposed, I should 
refer the broad race from Thingvallavatn. Different drawings of 
this species exist. A drawing which comes near to the Thing- 
vallavatn form has been published by O. Zacharias (1898. 
p. 716; 1899, p. 85). He gives some measurements which do 
not differ much from those of the Icelandic form, the most 
striking divergence being the length of the setae. Also Bruno 
