striped, and sometimes provided with short spines; often distinctly
porous. The anterior half is composed of 3 equatorial and 3 apical
plates, the latter being continued into the horn-like process. The
posterior half is composed of 3 equatorial and one apical plate
continued into the posterior horn. The right posterior plate is
continued into a similar horn which may remain rudimentary or be
continued into a considerable process. Similarly the left posterior
horn is usually developed, but remains small. There may be from 2 to
3, 4, and 5 horns. Chromatophores usually present, green to yellow
brown.

Fresh and salt water.


Ceratium tripos Ehr. Fig. 25.

The body is somewhat triangular and bears three horns, two of which
are shorter than the other one and slightly curved upward.

Length, including the horns, 290µ.


[Illustration: Fig. 25.--_Ceratium tripos_.]


Ceratium fusus Ehr. Fig. 26.

Synonym: _Peridinium fusus_ Ehr.

The animal is very elongate, due to the presence of two long horns