ms: _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 68 mu; diameter 54 mu. 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_.

[Illustration: Fig. 23.--_Peridinium digitale_.]

Peridinium divergens Ehr. Fig. 24.

Synonym: _Ceratium divergens_ Kent.

The shell is spheroidal, widest centrally, attenuate and pointed
posteriorly; the anterior portion is armed with two short, pointed
horns, each of them having a toothed process at the basal portion
of the inner margin. They are frequently colorless and beautifully
transparent, the body being free from large opaque granules; again
they are colored brown or yellow. The nucleus is large and elongate
and finely granular. 75 mu long and 68 mu in diameter. Common.

[Illustration: Fig. 24.--Ventral and dorsal aspects of _Peridinium
divergens_.]

Genus CERATIUM (Schrank).

(Stein '78; Perty '52; Clap & Lach. '58; Bergh '82; Pouchet '83;
Gourret & Roeser '88; Buetschli '85; Kent '81; Senn 1900; Schuett '98.)

The general shape is a flattened sphere with three long processes
or horns. The cross-furrow is either spiral or circular; the
longitudinal furrow is usually wide and occupies the greater part of
the anterior half of the shell. The shell is thick, reticulate or
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