from The American Heritage® Dictionary of the English Language, 4th Edition
- n. The hard chitinous outer covering of the shell of many mollusks, especially freshwater ones, that protects the shell from the erosive action of water.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. The proteinaceous outer sheath of many molluscan shells.
from the GNU version of the Collaborative International Dictionary of English
- n. A chitinous membrane covering the exterior of many shells; -- called also epidermis.
from The Century Dictionary and Cyclopedia
- n. The horny epidermal investment of the shells of most mollusks.
The researchers compare it to the thin protein coating, called the periostracum, seen on the shells of other snail species.
 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.
Is their periostracum sticky or is the snails' mucus somehow involved in the process?
However, one problem with ink is that it isn't absorbed into the periostracum but stays on it and, sooner or later, even the most "permanent" ink flakes off.
Senescent or very large ones, usually found in slightly more extreme microenvironments i.e. on hydrothermal vent chimneys, seem to have most of their periostracum worn away, hence no spines.
The shell is very thin, almost like a trade-off between shell thickness and periostracum "hairyness".
The older shell above the varix had lost most of its periostracum and thus was white, while the younger shell below it still retained the yellowish-brown skin.
In the lot of 138 shells that I was measuring the other week, there were only 2-and this was one of them-that still had their yellowish brown periostracum.
Some are based on field observations: color patterns that remain invisible because clams possessing them either live buried in sediments or remain covered with a periostracum so thick that the colors cannot be seen .