Periostracum is the thin membrane-like organic layer that covers the outer surfaces of the shells of many snails [from Greek peri, around + ostrakon, shell].
Wherever the soil is low in calcium carbonate, land snails usually obtain their calcium carbonate by eating empty snail shells. The upper picture shows a shell of Mesodon thyroidus, a common northeast U.S. land snail. After the snail died, a smaller snail crawled inside the empty shell and ate a section of the shell behind the lip, leaving behind the membranous periostracum, part of which is sticking out (arrows).
Periostracum is usually what gives shells their colors. Old shells are almost always white, because they have lost their periostraca. The shell below is that of Neohelix albolabris, another northeast U.S. forest snail. Its white shell underlying the periostracum is visible at places where the latter has disappeared.
One interesting property of calcium carbonate that makes up snail shells is that its solubility increases as water temperature goes down. Geerat Vermeij1 noted that many snails that inhabit cold waters have very thick periostraca that help protect the shell from dissolving.
1. Geerat Vermeij. 1993. A Natural History of Shells. Princeton University Press.