Notes. 
580 
collected. In considering the table on pp. 582 and 583 it is necessary to 
remember that this year has been an abnormal one, that the months 
July and August have been very rainy, whereas the temperature of 
the air and therefore of the water also has been considerably below 
the average. The quantity and quality of the Plankton will therefore in 
all probability be different from that of a normal summer. 
The accompanying table may indicate the distribution and relative 
number of individuals of the various species found, [i. = isolated ; 
vr. = very rare ; r.= rare; rc. = rather common ; c.= common; vc.= 
very common ; a. = abundant ; — = wanting.] 
A few words may now be said on the constitution of the Thames 
Plankton, and on the distribution of the various species. As in all 
rivers, as yet investigated, Diatoms play an important part, generally 
exceeding the other forms in quantity to a great extent. In the 
neighbourhood of Kew this is not so noticeable, there being perhaps 
two or three Diatoms to one individual of the other groups, but as we 
go farther up the stream the former increase rapidly in number, so 
that at Windsor or Maidenhead the ratio has become about 20-1. 
Diatoms are especially abundant in the backwaters, although the 
number of different species is frequently less here than in the main 
stream. I am only able to account for the great decrease in the 
number of Diatoms at Kew by the above-mentioned hypothesis — the 
fresh-water species are dying off, whereas the marine species are not 
yet common at this point. The samples from Kew and even from 
Teddington altogether present a very different appearance to those 
collected higher up; at Kew the stream is full of mud, and when 
a sample is examined under the microscope nothing but organic and 
inorganic detritus is found in the field of view, the few living forms 
being very widely scattered in between these particles 1 . The samples 
from higher up, on the contrary, are practically free from mud, and 
the field of view is full of Diatoms, &c. Possibly this mud, occurring 
in the lower parts of the stream, is partly accountable for the decrease 
in number of the Diatomaceae. 
Fragilaria virescem is very common in all parts of the river 
examined; the other species, Fr . mutabilis, is far less abundant. 
Next, as regards abundance to the first-mentioned species comes 
Melosira varians ; M. moniliformis is found very commonly in some 
parts. Pleurosigma attmuatum is another very common form, especially 
1 Zimmer, ioc. cit., p. 4. 
