KEY TO FAMILIES OF DINOFLAGELLIDIA.

1. No crossfurrow; two free flagella Family _Prorocentridæ_

2. One or more cross-furrows 3

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

Cross-furrow close to Family _Dinophysidæ_
anterior end

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


KEY TO MARINE GENERA OF PROROCENTRIDÆ.

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 *_Exuviælla_
to the flagellum

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