THE PERIDINIALES OF PLYMOUTH SOUND. 193 
central; green masses, probably food material, at the anterior 
end. Length 0-028 mm. Occasionally in water samples May to 
July. 
(47) Gymnodinium filum n. sp. (Fig. 9). Body long and narrow, 
tapering to a thread-like point posteriorly. Anterior end conical. 
Transverse groove almost straight, longitudinal groove reaching 
about three-quarters of the way to the posterior end. Nucleus 
behind the centre. A dark brown mass (probably food remains) 
in front of and at the side of the nucleus. One specimen was 
found with no coloured body. Body clear and colourless with 
no striz. Length 0-065 mm. Rare in water samples July, 1915. 
Very fragile and easily collapses. 
Fic. 9.—Gymrodinium filumn. sp. x 466 
a side view, b ventral view. 
Genus SPIRODINIUM Schiitt. 
(48) N.R. Spirodinium fisswm (Levander) Lemmermann. Occurs occa- 
sionally in August and September in water samples. Conspicuous 
from its yellow colour and peculiar dorso-ventral flattening. 
Division in the free state was noticed in September. 
(49) N.R. Spirodinium spirale (Bergh) (Fig. 10). This species 
is exceedingly common in the water samples in many varieties. 
The typical form which agrees with Pouchet’s description and 
figure (1885, p. 67, Plate IV, Fig. 30) is usually much smaller 
than his specimens and generally colourless, although bright 
yellow examples are sometimes seen, such as Pouchet himself 
observed occasionally. The yellow examples are always blunter 
at the apex than the type which is pronouncedly acuminate. 
My specimens, including the yellow forms, measure generally 
0-04 mm. to 0-06 mm., whereas Pouchet (1883-85) gives 0-10 mm. 
as the typical size. The longitudinal striations are characteristic, 
