| 11. | The group is distinguished by having spore operculum falls off.
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| 12. | The dark edge of the operculum also fades with age.
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| 13. | The operculum of juveniles, however, is spirally marked.
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| 14. | An inconspicuous dusky spot is usually seen on the operculum.
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| 15. | Juveniles are generally lighter in colour, also possessing five dark operculum.
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| 16. | Most species have a dorsal operculum has a terminal nucleus.
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| 17. | The operculum only, and the edge of the mantle are yellowish.
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| 18. | The head end of the larval case is closed by an operculum.
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| 19. | The operculum of the case is covered with small pebbles.
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| 20. | The oval operculum is flat within, with four whorls.
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