tetraterpenoids (C₄₀), formally derived from the acyclic parent, Ψ,Ψ-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.