oral plate, two lateral apical plates, and one or two dorsal plates.
The two antapical plates frequently have a tooth-like process. The
bodies are colorless, green or brown.

Fresh and salt water.


Peridinium digitale Pouchet. Fig. 23.

Synonyms: _Protoperidinium digitale_ Pouchet; _Protoperidinium_ Bergh
p. p.; _P. divergens_ Peck.

The shell is covered with pits of large size. The posterior part
is hemispherical and surmounted by a single horn or spine. The
transverse furrow is very oblique, and its two extremities are united
by a sigmoid longitudinal furrow. The anterior half bears two spines
or horns of different size, and variable. The nucleus is spherical
or ellipsoidal and placed in the posterior half of the shell.

Length 68µ; diameter 54µ. Common.

Although the description of Pouchet's _P. digitale_ differs in some
respects from a careful description of the Woods Hole form, I think
the species are the same. The chief difference is in the single horn
of the posterior half; in Pouchet's form this is furrowed by a narrow
groove which runs to the S-shaped longitudinal furrow. In the Woods
Hole form I was unable to make out such a furrow. The flagella, also,
were not seen. This same form was pictured by Peck '95 as _P.
divergens_.