Genus PERIDINIUM Ehr. '32, Stein '83.

(Claparède & Lachmann '58; Bergh '81; Pouchet '83; '85; Gourret '88; Bütschli '86.)

The form is globular, ovoid or elongate, the apex frequently drawn out into a long tube. The transverse and longitudinal furrows are quite distinct, the former having often a spiral course about the body. The two halves of the body are similar, the posterior being somewhat shorter; the anterior half has seven equatorial plates, an 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.


Fig. 23.—
Peridinium digitale.
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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.

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. 75µ long and 68µ in diameter. Common.


Fig. 24.—Ventral and dorsal aspects of Peridinium divergens.
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