Mitra ancillides: Shell up to 24 mm (about 1 inch) in length, fusiformly-elongate and slender, moderately light in weight. Whorls about8 apart from a worn protoconch, spire whorls slightly convex, sutures distinct. First 3 postnuclear whorls fenestrate through bisecting spiral and axial striae of about equal strength, 4th and 5th whorl with spiral threads which are more prominent anteriorly to the sutures, and irregular longitudinal striae. Last 3 whorls with very fine spiral striae only, interstitial grooves occasionally minutely pitted; spiral striae number about 22-23 on the penultimate and about 60 on the body whorl, inclusive of about 10-12 slightly more prominent basal cords. Aperture narrow, about equal in height to the spire, smooth within, outer lip weakly thickened and simple. Columella not calloused, and with 5-7 oblique folds, siphonal canal twisted to the left, siphonal notch distinct. Existing specimens white to fawn in colour, ornamented with faded, yellowish fawn axial zones and bands.

Mitra ancillides: Apart from the type-specimen, we have seen only 3 other specimens, all in moderately fresh but beachworn condition, of this rare species. All the known specimens were collected at Anaa Island, and the exact range of distribution remains unknown. The species is closely related to M. nivea Broderip, which it resembles in sculpture and form, but it is appreciably smaller in the adult stage and more slender. M. ancillides is the type-species of Mutyca H. & A. Adams, by subsequent designation of Wenz (1943); the genus is considered synonymous with Mitra s. str.

Mitra ancillides: Anaa Island, Tuamotu Archipelago; Marquesas.