Dinoflagellates are a major component of the modern plankton. Of the 2192 species of marine free-living dinoflagellates presently described, an increasing number are being shown to produce resting cysts (probably hypnozygotes) within their life cycle. With rare exception, only the resting cysts fossilize, so they are of central importance in tracing the history of dinoflagellates through geological time. Cysts of many of the more common dinoflagellate species have distinctive morphologies allowing their geographic and stratigraphic occurrences to be traced. An ever-increasing number of taxa are also being shown to produce distinctive and geologically preservable cysts, potentially enhancing our knowledge of the diverse representation of dinoflagellates through time. Here the organic-walled cysts of 73 rare, poorly known or morphologically problematic marine dinoflagellate cyst species belonging to the orders Gymnodiniales (nine species) and Peridiniales (64 species) are reviewed, described and illustrated, and their stratigraphic ranges assessed. The names Echinidinium aculeatum and Echinidinium transparantum are validated.
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It is a pleasure to thank Chris Bolch for sharing images of the cyst of Gymnodinium trapeziforme , Karin Zonneveld and Chris Reid for loaning their holotypes for photography, Hyeon Shin and Zhun Li for providing images of cysts of Cochlodinium polykrikoides , and Thomas Verleye for providing images of Selenopemphix undulata and loaning his slides from the Black Sea for photography, Leen Degezelle kindly helped with the digital drawings. H. Gu was supported by the National Natural Science Foundation of China ( 41676117 ). MJH and VP acknowledge support from their respective Natural Sciences and Engineering Research Council of Canada Discovery Grants. Two anonymous reviewers are thanked for their helpful remarks.