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 flagella arise from the anterior end of the body. The sh.e.l.l 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 sh.e.l.l; the sh.e.l.l 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 sh.e.l.l Genus *_Gymnodinium_

With a distinct sh.e.l.l 2

2. Sh.e.l.l not composed of definite 3 plates

Sh.e.l.l 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 sh.e.l.l-surface

Cross-furrow well defined; Genus *_Glenodinium_ no markings

4. Two parts of sh.e.l.l equal or 5 nearly equal

Two parts of sh.e.l.l 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, with great Genus _Ceratocorys_ ledge-like walls

Cross-furrow wide, no ledge-like Genus _Oxytoxum_ walls

* Presence at Woods Hole indicated by asterisk.

KEY TO MARINE GENERA OF DINOPHYSIDae.

Diagnostic characters: The cross-furrow is above the center of the body, and its edges, as well as the left edge of the longitudinal furrow, are usually produced into characteristic ledges; those of the cross-furrow usually form great funnel-like anterior processes, while those of the longitudinal furrow usually form great, lateral, wing-like processes ornamented by ribs and other markings.

1. Without sh.e.l.l; longitudinal Genus *_Amphidinium_ furrow may open & close

2. With sh.e.l.l; longitudinal furrow 3 unchangeable

3. With distinct apical funnel 4

No apical funnel Genus _Phalacroma_

4. With great wing-like ledge 5

Ledges very small; body long, Genus _Amphisolenia_ needle-like

5. Ledge of longitudinal furrow 6 extends to posterior end