Definition of "periostracum" []

• The hard chitinous outer covering of the shell of many mollusks, especially freshwater ones, that protects the shell from the erosive action of water. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "The researchers compare it to the thin protein coating, called the periostracum, seen on the shells of other snail species."
• "[2] This shell has an outer tissue-like layer called the periostracum that changes color as the bicuspid grows."
• "Gastrocopta that I've run across are often in more xeric habitats - perhaps there's a combination of fluctuating moisture in the soil and the snail's ecology some are burrowers that tends to get soil stuck to the periostracum."