646 Fritsch. — Further Observations on the 
siderable number of other Algae are common, however, 
Clathrocystis aeruginosa being most abundant. The following 
other forms were observed : — Pediastrum Boryanum , P. pertu- 
sum , Scenedesmus quadricauda , wS. acutus with vars. obliquus 
and dimorphus , Chlamydomonas Braunii , Lemmermannia 
emarginata , Closterium gracile , £ 7 . striolatum , Staurastrum 
paradoxum , Gomphosphaeria aponina , Phacus longicauda. — It 
seems probable, however, that longer backwaters may at their 
head show less of the character of a river Plankton than those 
discussed in the present paper ; the backwater at Molesey 
(see table, p. 640) even showed a greater contrast from the river 
in the Plankton from its head as compared with that from its 
mouth. 
The following are the main points brought out by the 
present paper : — 
(i) The Thames has a well-marked living Plankton all 
the year round. 
(ii) The periodicity of the Plankton (mixed Plankton — 
Melosira — Synedra — mixed Plankton) differs rather markedly 
from that observed in continental rivers ; Asterionella forms 
a minor phase during the winter months. 
(iii) The backwaters, although differing very markedly 
in quality and quantity of the Plankton from the river itself, 
always bear the stamp of a river Plankton. 
It is hoped during the next year to make a more complete 
study of the periodicity and also of the Plankton of the back- 
waters of the higher parts of the river’s course. 
University College, London 
July 6, 1903. 
