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Abstract
The circinate suture is a preformed, levorotatory excystment suture present on specimens of the acanthomorphic acritarch Circinatisphaera aenigma gen. et sp. nov. The operculum is frequently adnate, but when detatched bears a distinctive notch on its margin. The offset of the excystment suture and corresponding opercular notch is new for acritarchs and can be used for orientation. The descending, levorotatory circinate suture resembles paracingula of many fossil dinoflagellate cysts in spiral direction, but Circinatisphaera aenigma lacks other characters typical of dinoflagellates. Specimens exhibiting this excystment structure have been isolated from Libyan and Saudi Arabian Lower Silurian strata.