Analysis of the dinoflagellate generaTenuaandCyclonepheliumindicates that the type species ofTenua, T. hystrix,is assignable toCyclonephelium,and that other species presently placed inTenuashould be reattributed. For their reassignment, the new genusSentusidiniumis proposed and nine species are herein transferred fromTenuatoSentusidinium(two provisionally). Other proposed new combinations includeAptea anaphrissa (ex Doidyx), Cyclonephelium dubium (ex Tenua)andHemicystodinium? taugourdeaui (ex Tenua).A revised diagnosis forCyclonepheliumis presented and the species presently attributed to that genus are listed.