periostracum
/ˌpɛrɪˈɒstrəkəm/Zoology
noun
The outer horny or fibrous organic covering of the shell of a mollusc or brachiopod.
Origin
Mid 19th century; earliest use found in Philosophical Transactions of the Royal Society of London. From scientific Latin periostracum from peri- + ancient Greek ὄστρακον.