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 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