rhe Pen cil n iece of Sutton Park , Warwickshire. 193 
the one figured by Lemmermann from certain of the lakes of 
Sweden. 1 
In both 1907 and 1908 Cercctium hirundinella formed its 
characteristic horned resting cysts in September. The increasing 
size of these cysts during their development causes a dislocation of 
the cell-wall at the equatorial groove. They apparently rest in the 
mud at the bottom of the pool from the end of September to the 
beginning of April. 
Fig. 25. Ceratium hirundinella, O. F. Mull. A and B three-horned form ; 
C and D, four-horned form ; E, three-horned form with resting cyst. D is a 
ventral view ; all the other figures are dorsal views. All x 200. 
The Periodicity of the Peridiniea: in Bracebridge Pool. 
Altogether six species of Peridiniese were observed in the 
plankton of this pool, but two of them— Glenodinium pulvisculns 
and Peridinium minimum —occurred only in very small numbers in 
August and September, 1908. 
The other four were conspicuous, and three of them dominant 
constituents of the phytoplankton. The relative frequency of these 
four species has been plotted out on the accompanying chart (Fig. 
26), and it will be noticed that for the years 1907 and 1908 there is 
a very striking correspondence in the curves. There are two cold- 
water species and two warm-water species, attaining their maxima 
in definite sequence each year. 
The first one to show activity is Peridinium anglicum, which 
begins in the late autumn, and by January is both numerous and 
active. Its numbers increase gradually until its great maximum in 
April. In the meantime, P. aciculiferum has appeared, attained a 
1 Lemmermann in Archiv. for Botanik utgifv. af K. Sv. Vet.-Akad. 
Bd. 11., No. 2, 1904, t. 2, f. 48. 
