Calciodinellum operosum Deflandre 1947, type genus of the family Calciodinellidae, and eight new morphotypes of calcareous dinoflagellates have been discovered in Quaternary marine sediments from the North Atlantic and Western Indian oceans, Mediterranean and Caribbean seas. Six of these have been isolated alive and studied in incubation experiments; so far two types have been germinated to produce motile thecate dinoflagellates identified as Peridinium trochoideum and Scrippsiella sweenyae; four others have produced athecate hypnoid protoplasts of a type commonly seen in cultures of those species among others. We conclude that Quaternary calciodinellids are peridinioid resting cysts having apical archeopyles and are produced by a small monophyletic group of dinoflagellates with the unique capability of primary calcification. Mesozoic and Tertiary calciodinellids probably are homologous with these Quaternary types.