Assertion: oyster's shell | /r/MadeOf | chitin
Facets:
Source sentences:
text: Most shells that are found on beaches are the shells of marine mollusks, partly because these shells are usually made of calcium carbonate, and endure better than shells made of chitin.
text: This has a single, "limpet-like" shell on top, which is made of proteins and chitin reinforced with calcium carbonate.