o-ventrally (fig. 17, section). An oral
furrow is present on the ventral side and the two flagella originate
in it (fig. 17, at left). The vacuole is on the left side. Food
vacuoles are present in the posterior part. The nucleus is central.
Movement creeping.

Fresh and salt water.

Anisonema vitrea (Duj.) Fig. 17.

Synonyms: _Tropidoscyphus octocostatus_ Stein '83; _Sphenomonas_ Kent
'81; _Ploeotia vitrea_ Senn 1900.

With the characteristics of the genus. It differs from freshwater
forms in having eight furrowed surfaces running somewhat spirally
from the posterior to the oral end. Length 50 mu; width 23 mu. This
attractive flagellate was quite common in decaying algae at Woods
Hole; its shaking movement, its peculiar furrowed surfaces, and,
above all, its perfectly transparent, vitreous appearance, were well
described by Dujardin. Stein's _Tropidoscyphus octocostatus_ is a
fresh-water form which may possibly be a distinct species, especially
as it is described with both flagella directed forwards.

[Illustration: Fig. 17.--_Anisonema vitrea_.]

Genus DISTEPHANUS Stoehr.

An aberrant flagellate bearing a single flagellum and a silicious
skeleton resembling those of the Radiolaria. The skeleton consists
of two rings of different diameter parallel with one another and
connected by silicious bars. From the wider ring half a dozen bars
radiate outwards and a similar number of short thorn-like bars point
inwards obliquely. The color is yellow, and except for the flagellum
the form might easily be mistaken for a Radiolarian, as has been the
case repeatedly.

Distephanus speculum Stoehr.

_Dictyocha speculum_ Stoehr; _Dictyocha_ Auc.

With the characters of the genus.

A single specimen only of this very interesting form was seen at
Woods Hole. It occurred in a collection of tow made near the end of
the wharf during the evening.

KEY TO FAMILIES OF DINOFLAGELLIDIA.

1. No crossfurrow; two free flagella Family _Prorocentridae_

2. One or more cross-furrows 3

3. Cross-furrow nearly central Family _Peridinidae_
(cf. _Oxytoxum_)

Cross-furrow close to Family _Dinophysidae_
anterior end

Several cross-furrows Family _Polydinidae_
and flagella (One genus, _Polykrikos_.)

KEY TO MARINE GENERA OF PROROCENTRIDAE.

Diagnostic characters: The transverse furrow is absent and the two
fla