gella arise from the anterior end of the body. The shell may be
bivalved.

1. No tooth-like process dorsal Genus *_Exuviaella_
to the flagellum

2. With tooth-like process dorsal Genus _Prorocentrum_
to the flagellum

* Presence at Woods Hole indicated by asterisk.

KEY TO MARINE GENERA OF PERIDINIDAE.

Diagnostic characters: The cross-furrow is nearly central (see,
however, _Oxytoxum_); the body may or may not have a shell; the
shell may or may not be composed of distinct plates; the plates are
distinguished as _equatorial_ (_i.e._, bordering the cross-furrow),
_apical_, and _antapical_, while still another, the "rhombic plate",
may be present, extending from the cross-furrow to the apex.

1. Without distinct shell Genus *_Gymnodinium_

With a distinct shell 2

2. Shell not composed of definite 3
plates

Shell composed of definite plates 4

3. Cross-furrow replaced by Genus _Ptychodiscus_
thin-skinned band

Cross-furrow well defined; Genus _Protoceratium_
reticulate markings raised
on shell-surface

Cross-furrow well defined; Genus *_Glenodinium_
no markings

4. Two parts of shell equal or 5
nearly equal

Two parts of shell very unequal 11

5. With transverse flagellum in 6
a distinct furrow

Transverse flagellum not in a 10
furrow

6. With horns, or with wing-like 7
processes

Without processes of any kind 9

7. Processes small, wing-like, Genus _Diplopsalis_
around flagellum-fissure

Processes horn-like 8

8. Anterior part with 7 equatorial Genus *_Peridinium_
and 1 rhombic plates

Anterior part with 5 equatorial Genus _Gonyaulax_
and no rhombic plates

Anterior part with 3 equatorial Genus *_Ceratium_
and no rhombic plates

9. Anterior part with 14 equatorial Genus _Pyrophacus_
and 1 rhombic plates

Anterior part with 7 equatorial Genus _Goniodoma_
plates

Anterior part with 4 equatorial Genus _Amphidoma_
plates

10. Apical extremity drawn out Genus _Podolampas_
into a tube

Apical extremity not drawn out Genus _Blepharocysta_
into a tube

11. Cross-furrow deep, wi