Cuttle-fish Bone (Cuttle-fish Shell, Os Sepioe) is the internal shell of the cuttle, Sepia officinalis, Linne (N.O. Dibranchiata).

A large mollusc common round the coast of Great Britain and abundant in the Mediterranean and Adriatic Seas, and in the Indian Ocean. The shells of dead animals are thrown up by the sea. Cuttle-fish shell is usually about 20 cm. long, 7 cm. wide, and 2 cm. thick, oval, convex on both sides. The animal consists of a flattened, ovoid head with two lateral extensions, and ten long tentacles surrounding the mouth, which is provided with two large powerful horny beaks. The shell is contained in the muscular mantle. It consists of a hard, concave chitinous coat, upon the concave side of which layers of calcium carbonate have been deposited. Its chief constituents are calcium carbonate (80 to 85 per cent.) calcium phosphate, sodium chloride, and 10 to 15 per cent. of organic matter. It is chiefly used as an ingredient in dentifrices, but has been given internally for sprue and dysentery.