i66 
Notes. 
Melosira arenaria, Cymbella gastroides and Pleurosigma attenuatum , of which three 
were not observed in the Plankton of the Trent at all. These remarks will suffice to 
show the difference in the constitution of the Plankton both as regards quantity and 
quality in the two rivers under discussion. 
We must now consider the resemblances between the Plankton of the Thames 
and Trent, and these are very marked. In the first place, the filamentous Diatoms, 
Melosira and Fragilaria, are important constituents in both cases. Further, both 
rivers have a number of species characteristic of Potamoplankton in common, viz. 
the species of Synedra and of Cymatopleura, Cyclotella operculata , Nitzschia sig- 
moidea , Surirella splendida , Campylodiscus noricus, Pediastrum Boryanum and Scene - 
desmus quadricauda. Apart therefore from a certain difference in composition and 
a general decrease in number of individuals, the Trent may be said to possess a 
typical river Plankton 1 , the nature of which is similar to that of the Thames. Lower 
stretches of the latter river (i. e. those at Richmond or Teddington) of course 
show more marked differences, owing to the influence of the tide 2 ; and it will be 
interesting to compare the lower portions of the Trent with them. 
A few interesting forms were found in the Trent Plankton. Ceratiam hirundi- 
nella has for the first time been observed in the Plankton of English rivers, as also 
Volvox globalor. The individuals of the former species were provided with one 
upper and three lower processes, of which the middle one was the longest, whilst 
the lateral ones were of unequal length. The occurrence of Bacillaria paradoxa in 
the Trent is of considerable interest, as I also observed it last year in the 
Thames near Teddington 3 ; it would seem as though this species could live in 
perfectly fresh water, although the number of individuals found in the two rivers is 
very small. 
We now come to the Plankton of the Cam, and in considering it we must bear 
in mind that we are dealing with a slow-flowing river, which is only tributary to the 
main stream, the Ouse. Owing to the inconsiderable current large numbers of 
aquatic plants are able to develop (cf. p. 165), and this point has already led me to 
compare the Cam with a Thames backwater. In such a river all the Plankton 
probably develops on the leaves, &c., of the aquatic plants (which are for instance 
covered with a sediment of those Diatoms which occur so commonly in the Plankton); 
the rate of flow is probably not sufficiently strong to interfere with their development. 
In 1903 I was able to study the Plankton of a number of backwaters of the Thames, 
and in looking through the Cam material I was at once struck by the great similarity 
of the Plankton from some points of view. As in the backwaters, the quantity of 
individuals is much greater, although the number of different species (Cam 16, 
Thames 30, Trent 32) is markedly less than in a main river like the Thames or 
Trent. Diatoms, however, are by far the most dominant forms in the Cam, although 
1 i. e. the Plankton is dominated by the Diatoms, only a few green forms being present in at all 
sensible numbers. 
2 Cf. the table in my algological note III. From all that I have seen it seems that the in- 
fluence of the tide is perceptible considerably above Teddington Lock ; the Plankton even at 
Hampton Court is not so rich in green forms as in the higher reaches of the river. This is only one 
of the many problems that the Thames and other big rivers present. 
3 Cf. Fritsch, Further observations, &c.; loc. cit. pp. 638, 639. 
