Axial ribs strong and well defined, extending from suture to suture on spire whorls, and almost or quite to the base of the last whorl; number 8-11 on the body whorl. Spiral lirae stronger and coarser than leucothea, these being well developed even below the suture, periphery with a single lira which is markedly stronger than the others where it crosses the ribs. First lira below suture with a series of fine denticulations. Whorls tend to be more convex than in leucothea.
Typical coloration yellowish or white, with the subsutural region, a band around the middle of the body whorl, and usually another just below this, chestnut brown; this colour usually suffuses the intervals between the axial ribs as well. Occasional shells are completely chestnut, save for the crests of the ribs in the peripheral region, which are pale. Aperture usually deep violet, sometimes violaceous-brown. In the Cape Vidal-Mission Rocks race white examples predominate to the almost complete exclusion of coloured shells, although uniform flesh and brown-marked individuals do occur; in white shells the aperture is usually colourless but sometimes vivid violet in colour, and this may impart a slight violaceous tint to the exterior as well.
Dimensions: 31,5 x 15,3 mm, 30,3 x 14,5 mm (Mozambique); 22,4 x 11,1 mm, 21,9 x 11,6 mm (Cape Vidal); 28,3 x 13,1 mm, 26,7 x 13,3 mm (Umhlali).
As the above measurements indicate, Umhlali examples vary somewhat in width; in plump specimens the axial ribs are weaker than in narrow ones, and sometimes become obsolete towards the back of the outer lip. Such individuals show a certain superficial resemblance to the Pacific Peristernia striata (Gray), which was in fact listed from Natal by Melvill (1891:408).

The presence in Natal of two closely-allied forms of Peristernia was acknowledged by Barnard (1959), who identified the more brightly coloured one as nassatula (Lamarck), the dull or white one as leucothea Melvill. However, while these two forms can certainly be regarded as valid taxa, they can be accorded no more than subspecific rank. Furthermore the more vividly marked form differs significantly from nassatula and is clearly referable to the Red Sea forskalii (Tapparone-Canefri).
The position is as follows. From the eastern Cape ranging north through Pondoland to the south coast of Natal individuals are completely white in colour, with markedly weak sculpture (particularly in the southern part of the range). This is subspecies leucothea. In the Durban area, however, it meets and hybridizes with the nominate subspecies, forskalii, which has its centre of distribution in East Africa. Such specimens show a tendency towards the development of a general brown tinge and a violaceous aperture. From the Umhlali district northwards, typical forskalii is to be found, members of this population showing vivid chestnut-brown zones, a violet aperture, and markedly stronger and rougher sculpture.
However, at Cape Vidal (Umpangazi) and at Mission Rocks (about 16 km south) in Zululand, a local deme with quite distinctive characters has developed. While individuals resemble forskalii in sculptural detail, they are smaller in size, and are usually pure white in colour as in leucothea, although the frequent presence of a violaceous aperture betrays their affinities. Despite the superficial resemblance to the southern leucothea, gene exchange with that subspecies is geographically not feasible. Most intertidal reefs in Zululand are separated by considerable expanses of sandy shore from their neighbours, and development of this local race can more feasibly be ascribed to the normal genetic drift so often found in isolated colonies. Trinomial recognition of the form is not warranted.
The relationship between forskalii and nassatula is a point that cannot be resolved here. The latter has been recorded from a number of western Indian Ocean localities, yet the available material seems to indicate an eastern Indian Ocean-western Pacific distribu¬tion, and Demond (1957: 321) in fact gives the range as 'from Cocos-Keeling Atoll in the Indian Ocean eastward through the Pacific . . .'. P. nassatula is larger, broader and more biconical than forskalii, with a sharply-keeled peripheral angle and fewer, stronger spiral ridges. However, immature examples from the Andaman Islands (Natal Mus.) are possibly intermediate, and Spry (see below) may well be correct in treating forskalii as a subspecies of nassatula.