1904-5.] The Plankton of Thingvallavatn and Myvatn. 1117 
(later on of two) thickened rings placed parallel to the valve faces,* 
as shown in fig. 3. At the same time we generally notice that 
the cell of the chain next to the auxospore-forming cell is in 
bipartition ; it has reached the prolongation stage. When the 
auxospore is fully developed, it almost immediately undergoes a 
division : between the half-globular valves a cylindrical part 
appears (fig. 4) and new flat valves are formed (figs. 5, 6). We 
now get a two-cellular chain, of which each cell has one globular 
and one flat valve. Contemporaneously with this process the above- 
mentioned next placed cell has finished its bipartition and has 
begun a new division. The auxospore chain continues its divisions 
in the ordinary manner (cf. Otto Mliller, 1883); often many- 
celled chains (fig. 7) with the auxospore valves at the ends have 
been found. The thickness of these chains is about 2-3 times 
that of the ordinary ones. The auxospores are often cohering to 
the ordinary chains until the auxospores become two-cellular, hut 
rarely later. 
The auxospore formation must begin rather suddenly ; in the 
sample of 14th December, no auxospores have been observed, while 
in that of 23rd January these are numerously observed and some of 
them already consist of two cells. In the sample of 18th February 
many-cellular auxospore chains occur, and such is also the case in 
the following samples. They do not become rare before May-June. 
Also in M. italica I have found auxospores ; but only rarely, 
and not at different stages; they occur in both samples from June 
1903. They are rather like those of the foregoing species, as will 
be shown by my figures (PL II. figs. 6-8), but there are some 
differences. ' They are globular, and appear in the same manner 
in a prolongated ordinary cell, but while the cell in M. islandica 
soon bursts, it is not so in M. italica ; on the other hand, the 
neighbour cells have often disappeared, the individual then only 
consisting of a much prolongated empty cell with a globular auxo- 
spore placed in the thin- walled part nearest to one of the valves. 
A further distinction from the above-mentioned species is the 
connecting ring, which is not always parallel to the valve faces, 
but often more or less oblique. As I have seen only the unicellular 
* Holmboe (1900) has, p. 17, mentioned such auxospores in M. granulata, 
and given figures of them (figs. 2 and 3). 
