Siliceous spicules of endoskeletal dinoflagellates form a very small part of the siliceous microfossil assemblage from Neill Island, Ritchie’s Archipelago, Andaman Island and Nicobar Island. They are represented here by a solitary genus, Actiniscus, with two species – Actiniscus pentasterias and Actiniscus elongatus. Light microscopy, supported by scanning electron microscopy, illustrates its remarkable symmetry and geometry. Though biostratigraphically unimportant, this genus has considerable value in deriving palaeoecological interpretations. Their association with age-diagnostic calcareous nannofossil taxa indicates that the glauconitic mudstones of the Sawai Bay Formation of Nipple Hill and East Coast sections are Late Miocene in age, equivalent to the Discoaster berggrenii Subzone (Subzone CN9A) corresponding to the lower part of the Discoaster quinqueramus Zone (Zone NN11).

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