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shell structure Shell growth is similar in Mollusca and Brachiopoda. The shell is divided into an outer periostracum, which is organic, and an inner ostracum. Both parts are secreted by the mantle and shells grow by marginal increments which are commonly visible on the exterior as growth lines. Shell thickening takes place and in some groups accessory or additional shell material may be laid down. External ornament, e.g. ribs, often reflects the shape of the mantle edge or structures contained within it and quite complex patterns of ornament are sometimes produced.