The Peridiniece of Sutton Park, Warwickshire. 195 
great maximum, and disappeared again, all within the short time of 
two or three months. In 1907 this northern species appeared in 
February, reached its maximum abundance about the middle of 
March (temp. 5-9 n C), and had completely disappeared before the 
end of that month. In 1908 it had a similar but slightly longer 
activity, appearing in January and again attaining its maximum in 
March (temp. 5T n C). 
P. anglicum very rapidly dimishes after its great maximum in 
April (temp, about 9°C), and completely disappears by the end of 
May. 
Of the warm-water species, Ceratium hirundinella appears first 
in April, gradually rising to a maximum in July or August (temp. 
16—20 n C), and quickly disappearing by the end of September. 
Peridinium cinctum var. Lemmermanni begins its activity in 
April or May, attains its great maximum in August or September 
(temp. 17—19 n C), and then rapidly and completely disappears by 
the end of October or November. 
It will be noticed from the chart that all four species disappear 
very rapidly, the curves all having very steep downward slopes. 
This is to be attributed in every instance to the rapid formation of 
resting-cysts at the close of the active period. In two species the 
encysted stage is induced by the vernal rise of temperature, and in 
the other two species by the autumnal fall of temperature. 
Horned cysts were only observed in Ceratium hirundinella , and 
the formation of lunate cysts, with produced and spinate extremities 
such as those described and figured by Stein 1 in Peridinium tahulatum 
and by Schilling 2 in Glenodiuium cinctum, was not seen in any 
species of Peridinium in the two years these organisms were under 
observation. 
A curious irregularity is noticeable towards the end of 1908. 
In this year Peridinium anglicum appears to have had a resting 
period of only a little over three months. It recommenced its 
activity in September and very rapidly attained a great maximum 
in November, which it maintained throughout December, 1908, and 
January and February, 1909. This deviation from the general 
seasonal periodicity of the organism (as shown during the two 
previous years) is partly due to the early cold of the autumn of 
1908, which was continued right through the winter. At the same 
1 Stein, Der Organismus der Infusionsthiere, Abtli. III., Hiilfte 
II., 1883, t. 12, f. 20—28; t. 13, f. 1—5. 
2 Schilling, l.c. t. 2, f. 28—30; t. 3, f. 3—4b. 
