i86 
G. S. West. 
the smallest species of the genus, the cells having a length of 26— 
27[x and a breadth of 22/x (Fig. 20, H and I.) The examples agreed 
well with both Stein’s and Schilling’s account of the species, and 
also with specimens which occurred abundantly in the plankton of 
Tanganyika and Lake Nyasa. 
III.— Peridinium aciculiferum, Lemm. 
This interesting species was first described by Lemmermann 1 
as occurring near Berlin. In a later paper on the phytoplankton 
of the “ Muggelsee ” 2 he records it as occurring from February to 
April, with water-temperatures varying from 2-9 n C to 12T n C. 
Ostenfeld has also found the species in the plankton of Thingval- 
lavatn in Iceland. 3 It appears in that lake in January and 
June, having its maximum from February to March with a water- 
temperature of l n C. 
In the plankton of Bracebridge Pool, P. aciculiferum occurred 
from February to March in 1907, and also from February to March 
in 1908. In each year it attained its maximum in March (with 
water-temperatures of 5 , 0°C and 5-l n C) and completely disappeared 
before the end of the month. It has thus but a fleeting existence 
in the plankton, although in March it is one of the dominating 
constituents. 
The occurrence of this species in great abundance in the early 
spring plankton of a Midland pool is of great interest, as it is 
essentially a cold-water species, and possibly a northern type. 
Curiously enough, I have not yet found any trace of it in the spring 
plankton of the English Lake District, nor in any Welsh or Scottish 
lake. 
P. aciculiferum appears to be well-marked by its three posterior 
flattened spines. In the general disposition of its plates it agrees 
closely with P. umbonatum Stein, and this fact has recently induced 
Lemmermann to place it as var. aciculiferum of that species 1 . In 
the Midland specimens the plates were very indistinct. They could 
always be discerned in the empty cell, but were generally too faint 
for their exact disposition to be ascertained. The cells are some¬ 
what more compressed than in the majority of the species of this 
genus, and I find considerable variability in the general proportions 
1 Lemmermann in Ber. Deutsch. Bot. Ges., 1900, XVIII., p. 28. 
2 Lemmermann. “ Brandenburgische Algen., II.,” Zeitschrift fiir 
Fischerei, XL, 1903, pp. 86—90, 1909. 
3 cf. Ostenfeld & Wesenburg-Lund in Proc. Roy. Soc., Edin., 
Vol. XXV., Part XII., 1906, pp. 1126—1128. t. 1, f. 11 20. 
