There are as many as thirty or forty species of the Turban-shell.
The Marble Turban, _Turbo marmoratus_, is the largest, being as much
as four inches across; it is found in the Indian Ocean. The colour
of this shell is of a brownish green, of greater or less intensity.
It is ornamented by eight or ten narrow transverse belts, consisting
of a series of white or brownish spots. This is one of those shells
whose substance, after the outer coat is removed, is of the nature of
mother-of-pearl.
The _Turbo littoreus_ (the Shore Turban), is the well-known periwinkle,
with which our rocky coasts abound.