For instance, the colour purple, which had been a mark of aristocracy and prestige since ancient times in Rome, the Middle East and Egypt, was especially expensive and difficult to produce—the dye used, known as Tyrian purple, was made from the glandular mucus of certain molluscs. Its extraction was variable and complicated, and dependent on the availability of the very specific type of shell (actually two types, now known the one as Bolinus brandaris, and the other as Hexaplex trunculus, nowadays classified within two different genders) from which it was extracted.