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The sequence of changes following experimental removal of a central region of one valve of Mytilus edulis included: (1) the secretion of periostracum similar to one of the three layers of normal periostracum; (2) calcification on the periostracum in a pattern corresponding to the form of the periostracal surface; (3) the deposition of prismatic and nacreous shell layers also present in normal shell; and (4) the frequent formation of a complex prismatic-like shell. The deposition of all layers was carried out by a region of the mantle which normally deposits only nacreous shell.