634 Fritsch . — Further Observations on the 
The following observations on the periodicity of the Thames 
Plankton were all made on the stretch of river lying between 
Teddington Lock and Kingston, and were carried out on seven 
separate occasions, from October to the beginning of July, at 
intervals of from one to two months. More frequent visits 
would have been desirable, but were prevented by the state 
of the river ; unfortunately it is also impossible to undertake 
any further investigations during this year, and I have therefore 
added the results of my dredging in July of last year (Fritsch, 
’02), to complete the table to some extent. These latter were 
obtained from the river below Teddington Lock, the Plankton 
of which, however, shows no marked difference from that above 
the lock. All the samples were taken from the surface-layers 
only. 
In the samples, collected in October, abundant life was still 
present ; in addition to a number of green forms, of which 
Pandorina morum and Pediastrum Boryanum were most 
frequently met with, the Diatoms, Melosira varians and 
Fragilaria virescens , were exceedingly common. The three 
species of Surirella and Stephanodiscus Hantzschianus are 
also very characteristic of the Plankton at this time of the 
year, whilst other rather abundant forms are Synedra Ulna , 
Nitzschia sigmoidea , and Pleurosigma attenuatum . On the 
whole it is scarcely possible at this time to point out any 
one species as preponderating considerably over the others, 
although perhaps the two filamentous Diatoms are the most 
striking forms in the samples collected in this month. In 
December the green and blue-green forms had become ex- 
ceedingly rare, whilst the Diatoms, as already stated above, 
although still occurring in appreciable numbers, are to a con- 
siderable extent present as empty frustules. Melosira varians 
is still very abundant, whilst Fragilaria virescens is less 
evident, and the species of Surirella are decreasing in numbers. 
The most noticeable point about the Plankton at this time 
of the year, however, is the appearance of Asterionella 
gracillima in small numbers ; no trace of it was observed in 
the October samples. In February the Plankton practically 
