1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1113 
they belong to the bottom flora and have been carried away 
accidentally by the rivulets or the waves. 
Sphcerocystis schroeteri, Chodat. — This widely distributed 
plankton-alga is found in most of the samples ; it seems to have 
its max. in the summer (June) and the autumn (October). Most 
of the specimens agree well with the figures, p. 64 and p. 115, in 
Chodat’s paper (1902). 
Oocystis crassa, Wittr. — Following Sphcerocystis in its 
periodicity a species of Oocystis occurs. It .is very difficult to 
identify the species within this genus, in which the number of 
species has augmented greatly during the last few years, especially 
by descriptions of new forms by W. West and G. S. West and E. 
Lemmermann. The latter author has given a key to the plankton 
species of Oocystis (1904 a, pp. 106-108). G. S. West has recently 
(1904, p. 227) figured several species, and Chodat (1902, pp. 
189-191) has enumerated the Switzerland species. These three 
authors do not quite agree in their views of the definition and 
limits of the species. 
Lemmermann has created a new genus Oocystella , differing from 
Oocystis only by the existence of pyrenoids and the substellate 
chloroplasts, but West as well as Chodat admit the existence of pyre- 
noids in species of Oocystis ; it seems therefore unnecessary to have 
this separate genus. The species from Thingvallavatn has one pyre- 
noid in each chloroplast ; the chloroplasts are two or four in each cell, 
often tetrahedrally arranged ; the cells are four (rarely two) in a 
globular mucilage ; their shape is ellipsoid or ovate with subacute 
apices; length 22-26 g, breadth 16-20 /x. In my note (1904, p. 
235) on phytoplankton from a lake in South Iceland, I have drawn 
a figure of an Oocystis which is the same as the Thingvallavatn 
form;* the specimen drawn shows two chloroplasts in each cell, and 
the cells are not as subacute as in our form ; the pyrenoids are not 
visible. Of the Thingvallavatn form I have drawn a specimen with 
pyrenoids in the four chloroplasts (PI. I. fig. 8) ; this drawing 
comes near to G. S. West’s figure (1904, p. 227, fig. 97 D) of 
O. crassa , Wittr., but these latter have eight chloroplasts. It is 
upon this figure that I base my determination of the form, as the 
description given by V. Wittrock (1880) is very short and with- 
* I have named it O. lacustris, Chod. (?), which is evidently incorrect. 
