Protoconch large, prominent, translucent white, 1,5 convex whorls, first half-whorl deviated, merging gradually into adult sculpture, nucleus submerged. Teleoconch 4,5 whorls, strongly shouldered and turreted, flatly canali¬culate, flatly convex, constricted at base. Sutures deeply impressed. Sculpture of thin, fine, sharp-edged vertical axial ribs or ridges, becoming flattened with age, 9 on body-whorl, 12 on penultimate and next whorl, elevated above shoulders, scale-on-scale formation and reflected backwards, continued obliquely across top of shoulders to sutures as very fine raised lines, numerous fine growth lines between; transverse sculpture of numerous tightly packed and extremely fine rounded striae, crossed by microscopic growth lines. Umbilicus a slight indentation towards base of inner-lip callus. Aperture elongate-ovate, constricted either end, obliquely truncated posteriorly and with one low rounded tubercle, short broad open siphonal canal anteriorly; inner-lip reflected as strong callus on to body-whorl; outer-lip thin and sharp but thickened into heavy varix at rear, consisting of fine closely packed lamellae; seven fine low lirations internally; columella straight, two strong scarcely oblique plaits in centre, a third fold forms edge of canal. Colour off-white or grey white, aperture pure white. Dimensions. Holotype, length 12-6 mm, breadth 7-2 mm. Specimen is apparently fully mature.

This fine small species is closest to Trigonostoma vinnula Iredale, from New South Wales, having most features in common, but can be identified readily by the stark white aperture contrasting with the greyish-white colour of the shell; no specimen of T. vinnula has been observed which is other than a light shade of brown or fawn with a narrow white band round body-whorl. In addition this new species has vertical and not oblique axial ribs, also the sides of whorls and the outer-lip of aperture are far flatter in outline than the convex whorls and outer-lip of T. vinnula.

Distribution. Type locality, and S.W. of Mandurah, Western Australia (33° 05' S., 115° 18' E.), 220 metres.
Type locality. North-west of Bunbury, Western Australia (33° 10' S., 115° 18' E.), 201-228 metres.