Protoconch prominent, of 1,5 smooth shining convex whorls, first half-whorl deviated, nucleus well submerged. Teleoconch 4,5 convex, coronate, flatly shouldered whorls. Sutures deeply impressed. Sculpture of strong, rounded oblique axial ribs, 9 on body-whorl and penultimate, coronate at top of shoulders, and crossing flat space to sutures obliquely; ribs crossed by 9 fine transverse striae on body-whorl, 4 on penultimate, with a microscopic thread between each pair, whole sculpture crossed by microscopic growth lines. Umbilicus small and deep, partly covered by inner-lip callus. Aperture ovate, posterior end truncate, a short broad open siphonal canal; columella straight with 3 strong oblique plaits; inner-lip recurved as strong callus on to body-whorl; outer-lip regularly curved, sharp-edged and flared outwards; 10 lirations commence below edge of outer-lip and extend well into aperture. Colour fawn to light brown, with narrow white band round centre of body-whorl. Dimensions. Holotype, length 12-5 mm, breadth 7 mm. Normal size. Largest specimen examined 18 mm x 12 mm.

The closest congener to this species is T. diamantina nov., described in this revision from Western Australia. The only species within its range with which collectors could confuse T. vinnula is Trigonostoma bicolor (Hinds), stunted specimens of which occur in central New South Wales, formerly known as Trigonaphera interlaevis Laseron; these can be readily separated by their much larger umbilicus, and even though the red-brown band may be absent round body-whorl, the brown patches are usually present in the concave spaces on top of shoulders between ribs.

Distribution. 73 metres off Redhead, New South Wales (32° 57' S., 151° 45' E.), southwards to Twofold Bay, N.S.W. (37° 04' S., 149° 56' E.), 27 metres. Greatest depth recorded, 73 metres off Redhead as above. Appears to be confined to moderate depths, seldom seen on beaches. Type locality. Off Twofold Bay, New South Wales (37° 03' S., 150° 03' E.), 45 metres.