THE PERIDINIALES OF PLYMOUTH SOUND. 187 
Genus CERATIUM Schrank. 
(32) N.R. Ceratium platycorne V. Duday. Rare, occurred singly two 
or three times in the tow nettings. 
(33) C. bucephalum (Cleve). Occurred sparingly in tow nettings in early 
summer, more frequently in the late summer months. 
(34) C. tripos (O. F. Miill.). Occasionally in water samples and tow 
nettings. A variety which approaches the form lineata (EKhrb.) and 
which I have referred to this variety occurs more frequently 
(Fig. 1). This has a short and straight apical horn, the 
hind horn nearly straight and the right horn about one-third as 
long as the left. The usual markings are longitudinal striations 
from the apex to the girdle, sometimes also with reticulations. 
Fig. 1.—Ceratium tripos (O. F. Mill.) £. lineata (Ehrb.). x 466. 
Although apparently nearest to the form lineata, the apical horn 
is very much shorter—less than half the length from its apex 
to the girdle. 
(35) * C. arcticum (Ehrb.). Very rare, in tow nettings, 1915. 
(36) C. macroceras (Ehrb.) Cleve. Rare in tow nettings. 
(37) C. furca (Bhrb.). Occasionally, in tow nettings and water samples 
in summer. 
(38) C. fusus (Khrb.). The commonest Ceratiwm here. Occurs both in 
tow nettings and water samples and is often the only peridinian 
present in the winter. Maximum in October. 
GYMNODINIACE. 
By far the greater portion of the Peridiniales of this area belong to 
this group and are missed almost entirely by the tow nets, only a few 
of the larger forms being retained by them. 
