192 
G. S. West. 
have all made this error. As the Midland form differs considerably 
from typical P. cinctum in the size and shape of its apical plates, 
and has retained its features quite constantly for two years, I have 
named it as a variety. 
Diagnosis P. cinctum var. Lemmermanni. Cell, differt a 
forma typica in tabulis apicalibus epivalvse multe majoribus et in 
forma iis typicis non plane congruentibus ; etiam tabula equatoria 
dorsali multe majori. Long. 5g—70/x ; lat. 02—70/x ; crass, max. 
52 — 53/x ; diam. 58 —68/x. 
VI. — Peridinium minimum. Schilling. 
This minute species occurred only very sparingly in the August 
and September plankton of Bracebridge Pool in 1908, and most of 
the specimens seen were dead, empty cells. It has not previously 
been recorded for the British Islands. 
VII.— Ceratium hirundinella. O. F. Mull. 
This species, the most ubiquitous and variable of all the fresh¬ 
water Peridinieae, is a regular constituent of the summer plankton of 
Bracebridge Pool. It makes its first appearance in April and 
gradually increases to a maximum in July or August, after which it 
rapidly decreases, until by the end of September it has completely 
vanished. In this pool the maximum is not a large one and the 
numbers obtained in the tow-nets bear no comparison with the 
enormous multitudes of Peridinium anglicum or P. cinctum. It is 
the first time that this organism has been observed in a British 
pool of such small dimensions, but its existence as a decided 
summer form is strictly comparable with its general occurrence 
throughout Western Europe. 
Two distinct forms occur simultaneously, a three-horned and a 
four-horned form. The latter (Fig. 25 C and D) is a common form 
in the British lake-plankton 1 , and also in the plankton of the lakes 
of Central Europe, 2 but the former differs in general shape and 
proportions from any three-horned forms so far recorded from the 
British Islands. The only published form I have been able to find 
which approaches this three-horned form from Bracebridge Pool, is 
1 W. & G. S. West in Trans. Roy. Soc. Edin. XLI., Part III., 
1905, p. 494, c. Fig. 1 C and D ; in Trans. Roy. Irish Akad. 
XXXIII., Sect. B, Part II., 1906, p. 94, c. Fig. 6. 
2 Consult various papers by Lemmermann, Bachmann, Forti, v. 
Keissler, Wesenberg-Lund, Brunnthaler, etc., etc, 
