The structures of five new alkaloids are reported. Sceletium alkaloid A4 (3) is a new type of Sceletium alkaloid containing a tetracyclic ring system and N-formyltortuosamine (8) is a ring C seco derivative of 3. Three additional members of the 3a-aryloctahydroindole class are described by the structures of the phenolic base, 4′-O-demethylmesembrenone (12), Δ7-mesembrenone (7), and sceletenone (15). The latter constitutes the prototype of a monooxyaryl member of this class. A unified biogenetic scheme which accounts for the origins of the various ring systems of the different classes of Sceletium alkaloids is presented. © 1974, American Chemical Society. All rights reserved.