THE MICROPLANKTON OF PLYMOUTH SOUND. 155 
organisms. I have recorded this species by colonies instead of cells, as it 
was practically impossible to count the latter. 
Halosphera viridis comes next in importance, its swarm spores occurring 
oftener in the water samples than the spheres themselves. The other 
flagellates occur sparingly but belong to the genera recorded by others 
from plankton and are almost entirely missed by the nets. 
Oxyrrhis marina I have placed with the Peridiniales ; this species and a 
small species of Carteria, although not often found in the water samples, 
multiply freely in cultures where they are often found. The numbers 
obtained for flagellates, with the exception of Phawocystis, are much 
smaller than Lohmann’s. 
(1) Pheocystis Pouchetii (Hariot) Lagerheim. Begins to be common in 
the middle of May and continues till the middle of June, interfering 
with all the tow nettings. Rare at other times. Not many colonies 
get into the water samples. The unidentified flagellates are chiefly 
swarm spores, probably of Phwocystis. 
(2) Dinobryon sp. (cf. balticum (Schiitt) Lemm.). Rare in August in the 
water samples in small colonies. A minute species. 
(3) Carteria sp. A very small species, rare, in water samples only. 
(4) Trochiscia Clevei Lemm. Rare, September and May. 
(5) Halosphera viridis Schmitz. Not uncommon from September to 
February. Very frequent in summer, especially the swarm spores, 
usually swimming freely but sometimes still in the parent sphere. 
COCOSPHAERALES. 
‘* Nordisches Plankton,” Vol. 2. 
Pontosphera Huxleyi Lohmann. This is the only species found. It 
occurs occasionally in summer and in early autumn is sometimes abun- 
dant. 
Coccoliths of other species are very rarely seen. 
SILICOFLAGELLATA. 
‘“‘Nordisches Plankton,” Vol. 2. 
The usual two species occur fairly commonly in the water samples. 
(1) Dictyocha fibula Ehr. From September to December and fron 
March to September. Commonest in September. 
(2) Distephanus speculum (Ehr.) Haeckel. Throughout the year, except 
in mid-winter, rather more abundant than Dictyocha. Commonest 
in September and October. 
