Many studies on mollusc sterols were made before the advent of modern techniques such as gas chromatography and mass spectrometry, and several structures ascribed to such sterols were incorrect because the preparations lacked homogeneity.The Monoplacophora are extremely rare and the sterols have not been investigated. Cephalopods are believed to contain cholesterol but they have not been investigated in recent years. Among the Amphineura, Aplacophora have not been studied while Polyplacophora contain Δ7-cholestenol sometimes accompanied by cholesterol and brassicasterol.Among the least evolved pelecypods, two protobranchs studied contained cholesterol and one also had cholestenol. Septibranchia have not been investigated. A dozen species of filobranchs and eulamellibranchs have been found to have complex mixtures with the same predominant sterols. All sterols of the sea scallop have now been identified and the most recent are 22-trans-24-norcholesta-5,22-dien-3β-ol, and a novel C-30 compound, 29-methylisofucosterol, neither of which has been found elsewhere in nature. A primitive bivalve, the lamp shell, has an almost identical sterol composition to the modern pelecypods.Gastropods contain predominantly cholesterol and sometimes its precursor, desmosterol.