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Hyolithida (phylum Mollusca, class Calyptoptomatida) Order of pyramid-shaped calyptoptomatids which have conical embryonic chambers. The operculum possesses one or two pairs of muscle scars which are bilaterally symmetrical. The shell is externally undifferentiated, and internally it is non-septate. The order ranges in age from Lower Cambrian to Middle Permian. The main genus, Hyolithes, had world-wide distribution during the Cambrian, with over 300 species.