Notes, 
581 
in the surface layers. The species of Surirella are very typical 
of the Thames Plankton ; one or other of them will always be found 
in the field of view, when a sample is placed under the microscope. 
Campy lodiscus noricus is common in many parts. Forms like Coscino- 
discus radiatus , Melosira angulaia, Rhaphoneis Rhombus , Synedra 
Acus var. delicatissima , &c., become rarer as we go higher up stream. 
Amongst the green Algae the two species of Pediastrum were 
common in nearly all parts of the river. After that Closterium 
moniliferum , Cosmarium margaritiferum and Scenedesmus quadricauda 
are most abundant. Eudorina elegans is common higher up, and is 
probably one of those forms which dies off early when the water 
becomes brackish. Pandorina morum is found sparingly everywhere. 
The other species of Scenedesmus is much rarer than the first, and 
this is also the case with the other species of Desmids. Most 
probably the Thames water is too calcareous 1 to admit of a proper 
Desmid vegetation. This is also the case with the artificial waters at 
Kew, which yield very few Desmids. 
The blue-green forms are commoner in the lower parts of the 
river. Only the two species of Microcystis are present in any quantity. 
The others are isolated and local. Rather commoner are some of the 
Flagellates, especially Euglena viridis ; Phacus pleuronedes was also 
frequently found. Dinobryon is rare, but I think it probable that early 
in the year this and other Flagellates are much more abundant than 
they are now. No Peridiniaceae were observed, which is worthy of 
note. 
As to the difference between the samples taken from the surface 
and those taken from deeper layers, I have found that the green forms 
are less common in the deeper strata, and consequently the Diatoms 
preponderate the more. Some of these latter, however, are also much 
rarer, e.g. Pleurosigma (cp. table). 
The backwaters always present a much more flourishing and 
1 My friend, Mr. L. Guttmann, was kind enough to analyse two samples of 
Thames water, for which I desire to express my sincerest thanks. The results were 
as follows : — 
First sample from Kew : — 
3-o parts of Cl in 100,000 (most of this is due to Sodium Chloride). 
11.70 parts of CaC 0 3 in 100,000. (Total hardness = 19.13.) 
Second sample, from Maidenhead : — 
2-20 parts of Cl in 100,000. 
16-20 parts of CaC 0 3 in 100,000. (Total hardness = 22-02.) 
R r 2 
