All cones have a periostracum, though not usually by the time they reach a
dealers' list. Cowries do not have a periostracum, as the animal's mantle
can extend to cover the whole shell.
As for the function of the periostracum, it may well be camouflage. Large
cones such as C. betulinus and C. litteratus have a thick, rough
periostracum to which the sand in which they live adheres. This makes them
very much less visible in their habitat than examination of their bright,
cleaned shells would suggest. Many cones have tufted or ribbed periostraca,
and these surface features may be designed to act as anchors for other
organisms. Certainly, the thicker the periostracum the more Bryozoans,
Zooanthinarians, oysters and so on are to be found clinging to it and
providing shape-dispersal camouflage.
Another advantage may be mobility : sand-dwelling cones such as C.
tessulatus or C. eburneus have thin, smooth periostraca which are oily to
the touch when wet. This may well help them slide through the sand as they
cruise along in search of annelids and so on to eat.
The periostraca of thick-shelled predatory cones such as C. textile or
magus is thin and translucent, perhaps reflecting their greater ability to
defend themselves and thus not to have to rely on camouflage. On the other
hand C. geographus, while very dangerous to fish, has a thick, tufted
periostracum which is often covered in growths. This may well be for
camouflage against crustaceans - which are immune to its poison.