outer horny layer of the shell.
the outside layer or covering of the shell
a thin, horny covering on the exterior of many shells, often with hairs or wrinkles
The flaky leathery outer covering on shells that protects from eroding or dissolving.
The hard outer shell of an oyster Pfucata
The periostracum is a thin organic coating forming the outer-most layer of the shell of many mollusks. The periostracum "is composed of a type of conchin [or conchiolin] (largely quinone-tanned proteins) similar to that found in the epidermal cuticle."Brusca, R.C. & G.J. Brusca 1990.