636 Fritsch . — Further Observations on the 
consisted of Diatoms only ; the green species, mentioned in 
the table, were both only observed once, groups of green 
Pleurococcoid cells being somewhat more frequent. Of the 
Diatoms observed, about two-thirds of the individuals were 
dead ; Melosira varians , Fragilaria virescens , and the species 
of Surirella , had decreased very much in amount, whilst 
Asterionella gracillima is far commoner, being very character- 
istic of the Plankton at this stage. Its reign is apparently 
short, however, for samples collected in the middle of March 
showed a very great decrease in the number of individuals of 
this species; otherwise the Plankton remains very much the 
same, the two filamentous Diatoms still preponderating over 
the others. At the same time Synedra Acus , var. delicatissima 
is present in sensible numbers (cf. foot-note, p. 637). In May 
there is no trace of Asterionella , whilst Melosira varians is 
present in extreme abundance, large numbers of the chains of 
this Diatom always lying in the field of view under the micro- 
scope ; Fragilaria is relatively far less abundant. The only 
other common species is Synedra Ulna , the green forms being 
just as rare as in March. This latter feature may in part be 
due to the strong current in the river on this occasion, owing to 
the heavy rains (cf. p. 631). Samples collected a month later 
(June 3, 1903) showed an increase in the green forms (species of 
Pediastrum , Chlamydomonas Braunii, Closterium moniliferum ), 
whilst the relative number of individuals of Diatoms occurring 
is approximately the same as before, Melosira varians being 
by far the most abundant species. In the next month two 
species of Synedra (S. Uhta and S*. Acus , as well as its var. 
delicatissima ) develop to an extraordinary extent, Melosira 
being almost lost in contrast to the numerous needles of these 
species. Two months before we had a Melosira- Plankton ; 
in July we have an excellent example of a Synedra-P lank ton. 
The green forms are now rather abundant, and Synura Volvox 
is also frequently met with. 
Stated briefly, the relative development of the Plankton of 
the Thames at different times of the year is thus as follows : 
In October, Melosira , Fragilaria , Surirella , &c„ in almost 
