638 Fritsch, — Further Observations on the 
to such an extent as to characterize the Plankton of the Oder 
at any particular period. Melosira , being well provided with 
chloroplasts, would be likely to be able to assimilate freely 
before forms like Synedra , Fragilaria , &c., with far smaller 
chloroplasts, were capable of doing so, and its great develop- 
ment during the spring months may be explained on these 
grounds (cf. Zacharias, ’99, p. 27). To some extent Asterio- 
nella and Synedra may be said to have exchanged places in 
the Thames Plankton with regard to that of the continental 
rivers. It remains to be seen whether other European rivers 
show the same periodicity as the Oder and Danube, and 
whether other British rivers will follow the lines of the Thames 
Plankton. Our present scanty knowledge of the conditions of 
life of the Plankton of rivers makes it impossible to account 
for this divergence ; it may be that the difference of climate, 
already referred to above, has something to do with the 
matter. It does not seem likely that the height of the water 
at some times during this year will have had any effect on 
the general features of the periodicity of the Plankton, although 
the minor points may have been to some extent obscured. 
From what I have seen I do not consider it impossible that 
different portions of the river’s course, sufficiently distant from 
one another, may show variations in the periodicity of the 
Plankton, a point which I hope to examine more carefully 
next year. 
A few points regarding the Plankton of this part of the 
river still deserve mention. In addition to collecting samples 
from the main river, on several occasions some were also taken 
from a side-arm of the river between Tatham’s Island and the 
Middlesex bank. Those taken from this arm in last October 
contained a number of individuals oiBacillariaparadoxa, Gmel., 
in a healthy condition, the concertina-like movement of the 
individuals of the colony taking place in rapid succession. 
This was not the only marine form observed here at the time, 
Pleurosigma Fasciola (Ehrb.), Sm. and Navicula amphisbaena , 
Bory being also represented in small numbers. The peculiar 
point about their occurrence here is that a diligent search did 
