642 Fritsch . — Further Observations on the 
were observed in very small numbers. On the same day 
a slow-flowing arm a little further down the river was exam- 
ined ; here the green forms were rather more abundant, and 
in correspondence with this animal life more frequent than 
in the main stream. Asterionella gracillima was again 
observed here, and even in rather greater quantity than in 
the backwater; it would thus appear as though some of 
the forms, which are already wanting in the main stream, 
manage to maintain their existence for a somewhat longer 
period in some of the backwaters and slow-flowing arms 
on the river’s course (cf. also the small backwater at Walton, 
discussed below). 
The two backwaters at Walton, except perhaps for the one 
at Shepperton, are the most typical of those examined this 
year. The first (the ‘ Sale ’) is a broad pond-like arm of the 
river just below the bridge at Walton; its connexion with 
the main river is about 5 to 6 yards in breadth, but a very little 
way inside it broadens out very considerably. It is deep 
enough to allow of easy rowing, and in part was filled with 
a growth of Nymphaea , & c. In no part of the river was such 
a diversity of green forms and Flagellates found as here, 
whilst blue-green forms, curiously enough, were entirely want- 
ing. It is unnecessary to especially mention any of these 
forms, as they are sufficiently evident from a glance at the 
following table ; the latter also shows how the large majority 
of them are wanting in the main river. In the case of a form 
like Synura Volvox , which is abundant in the backwater, 
the entire absence in the river itself is very noteworthy. On 
the other hand Gonium pectoral e, which is occasionally met 
with in the river at this point, was not observed in the back- 
water. In this latter, however, the first member of Peridineae 
that I have as yet found in the Thames was observed, but 
even then only scanty in number of individuals. With regard 
to the Diatoms, Melosira and the two species of Synedra are 
represented in rather equal numbers, whereas the latter are 
the prevalent forms in the river outside. The almost entire 
absence of Campylodiscus noricus and the species of Surirella 
