1128 Proceedings of Royal Society of Edinburgh. [sess. 
When the number of our species in May begins to decrease, a 
number of thick-walled cysts appear, one of which I have drawn 
on PL I. fig. 20 ; the cell-wall consists of cellulose, like the wall 
of the Peridinium ; the cell-content contains starch and is rather 
condensed, but a nucleus of the ordinary Peridinian shape is clearly 
shown. I suppose that these cysts represent the resting stage of 
the Peridiniums. In June our species has become very rare, only 
some empty scales of the Peridinium stage and some cysts having 
been found. I presume that most of the cysts have dropped to 
the bottom. Then probably the life cycle of the species is as 
follows : in the summer and autumn the cysts rest in the bottom 
mud, then in January-Pebruary they rise in the plankton 
developed in the Gymnodinium stage and embedded in a large 
jelly ; the cells divide repeatedly in Pebruary-March, and in April 
form the Peridinium stage with cell-walls consisting of plates. 
Prom this stage we must imagine the cysts developing in such a 
manner that the cell-contents of the Peridiniums go back from 
the inner side of the cell-wall, and then secrete the new, thick 
homogeneous cell- wall, thus forming the cysts which sink from the 
plankton to the bottom. 
Length of the Peridinium, 35-40 g ; breadth, ab. 30 g. Lemmer- 
mann says (1900, p. 28) 32-42 g broad and 41-51 g long : thus 
his forms seems to be a little larger than the Thingvallavatn form 
In a later paper (1903, p. 109) he mentions our species as 
occurring in the months February to April, and quotes (pp. 86- 
90) the following tp. figures of the water, observed when the 
samples were taken, viz., 4‘25°, 2 , 9°, 6'5°, 4*8°, 8*5° and 12-1°. 
These figures correspond rather well to the tps. of the Thing- 
vallavatn, varying from 1° to 8 ‘5° in February- June. We 
accordingly must take our species as a cold-water Peridinium. 
( c ) Zooplankton. By C. Wesenberg-Lund. 
Heliozoa. 
Acanthocystis aculeata , Hertwig and Lesser. 
During the time from 31st March to 30th April I have found in 
the samples a small Heliozoan ; it was always rare, being 
