The inner layer of a shell, particularly in mollusks, that forms the interior surface opposite the exterior periostracum.
From endo- (Greek 'within') + straca (Latin, 'covering'). Coined in the 1800s when malacologists needed precise terms for shell layering.
Pearl farmers depend on this concept—when an irritant gets inside an oyster shell, the mollusk coats it with nacre from the endostracum layer, creating a pearl!
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