Patchy organic sheets often exist in the inner shell layers of endolithic mytilids. They are commonly distributed beneath damaged portions of the shell both in mechanical and chemical borers (Adula falcatoides, Botula fusca and many species of Leiosolenus). The shell damage results from the abrasion and/or corrosion during boring activity. Shell dissolution by calcium-binding mucus or by cold seawater undersaturated with calcium ions occurs only in regions where the periostracum has worn out. The shell microstructure of the repaired portions of 14 species of boring mytilids suggests that internal organic sheets protect the shell from dissolution and three shell repair patterns using the organic sheets evolved independently in at least two clades of boring mytilids.