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, 290 mu.

[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
at the extremities of the body. Color, yellow with chromatophores.
Length 285 mu; width 23 mu.

Both of these species are common in the tow and in the algae at the
edge of the wharf. Both of them are mentioned by Peck in '93 and '95.

[Illustration: Fig. 26.--_Ceratium fusus_.]

Genus AMPHIDINIUM Clap. & Lach.

The body is ovoid to globular and usually much flattened
dorso-ventrally. The anterior portion is very much reduced and is
somewhat head-like or cap-like. The longitudinal furrow extends
through the entire posterior body length and is apparently capable
of widening and narrowing. It is probably naked (see here Klebs,
Pouchet, Buetschli), although Stein maintained that there is a
delicate cuticle-like shell. Chromatophores of brown or green colors
present and usually grouped radially about a central amylum granule.
The nucleus is posterior.

Fresh and salt water.

Amphidinium operculatum Clap. & Lach. Fig. 27.

The body is oval and flattened. The transverse furrow is at the
extremity (posterior) of the body and the small portion, which is
thus apparently cut off, is the cap-like or operculum-like structure
which gives the name to the species. Klebs maintains that the two
furrows are not connected, but in this he is certainly mistaken,
provided we have the same species under consideration. Very common
about Woods Hole.

Length from 40 to 50 mu; width 30 mu; thickness 15 mu.

[Illustration: Fig. 27.--_Amphidinium operculatum_.]

KEY TO INFUSORIA.

1. With cilia Subclass _Ciliata_. 3

2. Without cilia (in adult state) Subclass _Suctoria_
tentacles

3. a. Without a specialized fringe of Order _Holotrichida_
large cilia (ad. zone)

b. With general cov