Regression equations for 11 species of genus Ceratium (subgenus Eucera
tium) were suggested. These equations allow to reduce the number of me
asurements at several times. The regression equation for the subgenus
can be expressed in the form V = 8.468 D0.72W2.05, where V - cell volu
me (mu3), D - diameter of the apical horn base (mu) and W - width of t
he cells body (mu). Average values of the main geometrical parameters
for 11 species are presented.