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 _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 * Presence at Woods Hole indicated by asterisk. KEY TO MARINE GENERA OF PERIDINIDÆ. 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