The Peridiniece of Sutton Park, Warwickshire. 189 
These are globular and formed singly within the adult cell, from 
which they soon escape by the rupture of the wall in the vicinity of 
the equatorial groove. Each cyst possesses a large parietal 
chromatophore with many lobes (Fig. 23 E and F), or two or three 
such chromatophores. From this stage the multiplication occurs 
in two ways. 
In the majority of observed instances the non-motile cyst 
divides, each individual then becoming an adult Peridinium and 
developing a wall of plates (Fig. 23 G and H). This new wall of 
plates is often evident before the two individuals have completely 
separated. Sometimes each of the new cells divides again before 
the formation of the tabulated wall, so that four individuals are 
produced from the original cyst. 
In other instances the cyst gives rise to a motile Peridinium- 
stage with a very thin wall. This at once escapes from the wall of 
the cyst (Fig. 23 J) and begins to divide while slowly swimming 
about (Fig. 23 K and L). Several divisions may occur during this 
motile stage, the resultant individuals each developing a strong wall 
of plates and becoming a typical adult P. anglicum. The flagella 
are more obvious during this division than at any period in the life- 
history of the organism, largely owing to the slowness of their 
movements. All the divisions which take place in the motile stage 
are in a transverse plane, so that many curious states can be seen 
in which the body is much elongated and furnished with two 
transverse grooves (Fig. 23 K and L). The only figures 1 can find of 
division of this nature are those given by Penard 1 of what he 
doubtfully considered as transverse fission in Gymnodinium viride. 
So far as I can ascertain, this formation of thin-walled non- 
resting cysts with the resulting rapid multiplication, both in the 
non-motile and motile states, is a new observation in the life-history 
of a thick-walled Peridinium. It is certainly the reason for the 
occurrence of prodigious numbers of active individuals of this 
species in the April plankton of Bracebridge Pool. 
The formation of thick-walled cysts was not observed either at 
the end of April or in May, when this species is rapidly disappearing. 
Thin-walled cysts continue to be formed up to the time of this rapid 
disappearance, and it would seem that the last generation of thin- 
walled cysts persists through the summer and the early autumn as 
the resting cysts. 
The marginal spines bordering the two antapical (or posterior) 
1 Penard in Bull. Soc. Botan. Geneve, 1891, t. IV., f. 21 ; t. 5,f. 8. 
