Bistolida owenii vasta: Medium sized (14-24 mm), oval, often inflated, sometimes extremely callused shells. The margins are rounded and bent up, the white base is convex. The teeth are more densely spaced and extend far across the base and the labrum. The fossula is slightly projecting, the anterior columellar teeth form 5-8 denticles, the peristome is distinctly ribbed. The dorsum is olive-green to pinkish tan, with a single narrow brown uninterrupted transverse middorsal band that is present also in juvenile shells. The paler dorsal zone is reduced to a yellowish window in the anterior third. The fine brown dorsal spotting is usually forming a distinct irregular blotch situated above the transverse brown middorsal band on the right half of the dorsum. An interrupted brown band reaching from the margin towards the right side of the dorsum in the posterior third is found in specimens from shallow water in the northern half of Madagascar and East Africa including Zanzibar and the Comoros. The marginal spotting is mostly very dense, often forming a dark confluent frame encircling the dorsum, especially in specimens from the south of Madagascar and southern Africa. The basal spotting is dense and fine. The aperture is curved behind, the extremities are rather callused, mostly pointed but sometimes blending with the callus of the margins. The subspecies is widely distributed along the African mainland, from Kenya to the northern Transkei, the Comoros and Madagascar. Reports of vasta from Mauritius probably refer to specimens of menkeana. Occasional specimens of owenii from La Reunion show resemblance to vasta by being rather saturate blue, with a somewhat oval shape. There are numerous conchologically intermediate stages between the "northern form" and the "southern form" of vasta.