The Peridiniece of Sutton Park , Warwickshire. 187 
of the cell during the great maximum in March. Length of cell 
38—48//.; breadth of cell 29—33// ; maximum thickness 22—25//. 
Towards the end of March thick-walled cysts are formed (Fig. 22 
C), which undoubtedly sink to the mud at the bottom of the pool, 
and there rest until the following January or February. These 
resting-cysts are ellipsoid with somewhat conical poles, and are 
evidently of the same nature as the thick-walled cysts described by 
Ostenfeld 2 , although not quite of the same form. The extra¬ 
ordinarily rapid disappearance of P. aciculiferum about the end of 
March must be attributed to the simultaneous formation of large 
numbers of these thick-walled resting-cysts. 
Fig. 22. Peridinium aciculiferum, Lemm. A, ventral view ; B, dorsal view ; 
C, thick-walled resting-cyst; D—F, thin-walled cysts; G, division of thin- 
walled cyst. All x 500. 
During the time of rapid increase of this organism in January 
or the early part of February, another type of cyst is formed (Fig. 
22 D—F). This is not a resting-cyst, but one which almost imme¬ 
diately escapes from the old cell-wall. It becomes invested by a 
large gelatinous coat and quickly undergoes division (Fig. 22 G). 
The two new cells rapidly lose the mucous investment and become 
typical Peridinia with a wall composed of plates. 
IV.— Peridinium anglicum, sp. n. 
In the plankton of Bracebridge Pool this species attained an 
enormous maximum in the April of both 1907 and 1908. It made 
its appearance some two or three months before P. aciculiferum, 
1 Lemmermann in Archiv. fiir Hydrobiol. u. Planktonkunde, IV., 
1908, p. 181. 
3 Ostenfeld and Wesenberg-Lund, l.c., p. 1128, t. 1, f. 20. 
