* 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