Tetraterpenoids ($\ce{C40}$), formally derived from the acyclic parent, $\ce{\uppsi\!,\!\uppsi\!-carotene}$ I by hydrogenation, dehydrogenation, cyclization, oxidation, or combination of these processes. This class includes carotenes, xanthophylls and certain compounds that arise from rearrangement of the skeleton of I or by loss of part of this structure. retinoids are excluded.