The Peridiniea of Sutton Park , Warwickshire. 185 
gradual diminution in the activity of the organism leading to a state 
of complete encystment in December, coincides with the continuous 
and prolonged autumnal fall of temperature and the cold period 
immediately following. The July minimum, accompanied by partial 
encystment, occurred both in 1907 and 1908 at the period of highest 
temperature. In 1907 the spring minimum occurred in May, 
seemingly as a result of a rather rapid rise of temperature, whereas 
the minimum in April, 1908, was without doubt, due to the cold 
wintry weather which occurred towards the end of that month. 
Thus, Glenodinium uliginosum, in the habitat examined, becomes 
completely encysted for a brief period in December after the 
autumnal fall of temperature, and much reduced in activity (with 
partial encystment) in July, attheperiodof highestwater-temperature 
(18-22 n C). Other diminutions in activity, such as those observed in 
the spring months of 1907 and 1908, may occur either as the result 
of a sudden cold snap, or of a pronounced rise in the temperature. 
The organism exhibits its greatest activity when the temperature 
of the water is between 12 n and 17°C, and the optimum temperature 
lies about 15° or 16 n C. 
The method of encystment is very simple. The old cell-wall 
is thrown off in two pieces, which often hang together for some 
time, and the protoplast rounds itself off (Fig. 20 E—G) The cysts 
are either globular or broadly ellipsoid, and the newly-formed wall 
varies much in thickness. 
I have not been able to satisfy myself that the chromatophores 
are small and numerous as described by Schilling. 1 It seems 
probable that one may get a false impression of numerous chromato¬ 
phores, owing to the repeated lobulation of a much fewer number 
of larger ones. 2 Also, it can be frequently observed in the young 
individual developed from the cyst (Fig. 20 D), that there are only 
two or three lobed parietal chromatophores. 
The dimensions of the Sutton Park specimens were as follows:— 
length of cell 36-43//,; breadth 34-38//; max. thickness 26-27// ; 
diam. of cysts 33-40//. 
II. —Glenodinium pulvisculus (Ehrenb.) Stein. 
This species occurred very sparingly in the plankton of Brace- 
bridge Pool during August, 1908. No cysts were observed. It is 
1 Schilling, l.c., p. 64. 
2 Consult the chloroplasts of Cosmarium Debavyi, as figured by 
Liitkemilller in Oesterr. botan. Zeitschrift, 1893, t.3, f.25. 
