The status of the writhing skinks of theLygosoma sundevalliigroup in the East Mrican Coastal Mosaic is investigated and a new species,L. mafianum, described from Mafia and Kisuju Islands off the Tanzanian coast. The most primitive species isL. lanceolatumBroadley 1990 of the Bazaruto Archipelago, which retains pterygoid teeth, a discrete supranasal, tricarinate dorsal scales in juveniles, four supralabials anterior to the subocular, lanceolate ear lobules and numerous subdigitallamellae.L. afrum(Peters 1854) of Zanzibar Island and the mainland is the only other species which may retain pterygoid teeth and usually a discrete supranasal. It has smooth dorsals, only three supralabials anterior to subocular, no ear lobules and much shorter digits thanL. lancelolatum. L. mafianumis primitive in retaining keeled dorsal scales throughout life, but lacks pterygoid teeth and has the supranasal fused with the anterior part of the nasal.L. pembanumBoettger 1913 is the most evolved species, having smooth scales throughout life, prefrontals fused with the frontonasal and the lowest number of subdigital lamellae.