428 MR. B. M, GRIFFITHS ON THE AUGUST HELEOPLANKTON 
contaminate the water. These observations agree with those made 
in large lakes, where Æudorina is a common constituent of the 
plankton. 
(c) The presence of Microcystis wruginosa and other Myxophyceæ seems to 
indicate contamination. In the Hurcott series of pools, Microcystis 
was not observed in the two higher pools of Hurcott and Podmore. 
It occurred in fairly great quantity in the lowest pool, Broad waters, 
which is exposed to contamination from the village of Broad waters. 
The pool is large and the stream strong, so that the contamination 
is not sufficiently great to prevent the growth of Pandorina. 
Spring Grove Lower Pool abounded with JM. «ruginosa and 
M. ochracea. These alge were so abundant that they choked the 
pores of the plankton-net in a few minutes. This pool receives 
a good deal of house-drainage. 
Harvington Hall Moat was also very much contaminated and 
contained enormous quantities of Aphanizomenon Flos-aque. Micro- 
cystis occurred only in small numbers in this pool. 
(4) Spring Grove Upper Pool and Wilden Pool had a dominant Proto- 
coccales flora. The former pool is supplied by springs, and is 
much shaded by trees. Wilden Pool contained the alg:e Crucigenia 
reniformis, C. apiculata, Scenedesmus Raciborskii, and Dictyospharium 
pulehellum, all of which are confined exclusively to this piece of 
water. The water-supply is derived from the River Stour, which 
is here canalized for a distance of about a mile. Numerous barges 
come up from the Severn at Stourport and down from the Black 
Country of South Staffordshire, and pass along the canalized river. 
It is possible that many chance algæ are brought into the water of 
the pool by this traffic. 
(c) Peridinieze occurred in Spring Grove Lower Pool and Stanklin Pool. 
In both cases Ceratium Hirundinella was present, but each pool had 
its own definite variety of this species (consult РІ. 34. figs. 16 & 17). 
Peridinium anglicum was found in large numbers in Stanklin Pool. 
Summary :— 
Dominant Alye. Conditions. Pool. 
VOLVOCACEX ...... Strong stream and little contamination. Tlurcott. 
Podmore. 
Broadwaters. 
Island. 
PROTOCOCCALES .... Spring or river-water ..... Е Spring Grove Upper. 
Wilden. 
Мухорнуусем...... Much contaminated ............... Spring Grove Lower. 
Harvington Hall. 
PERIDINIEÆ, ....... Slight contamination. ,.............. Stanklin. 
