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.