Shell elongate-fusiform, up to 86 mm in length, width 28-30% of shell length, spire rather long (46-51% of shell length). Protoconch not preserved in type specimens. Teleoconch with 9-10 flat-sided smooth whorls which are transversely impressedly striated, striae finely punctured, 6 spiral striae on late spire whorls, 17-23 spirals on body whorl (not all striae distinct); base of shell including the siphonal fasciole only shallowly grooved. Aperture rather narrow (width = 19-20% of height); columella with 5 plaits which decrease in size anteriorly; inner lip slightly elevated only at the anterior half of aperture in fully mature specimens; outer lip thin, with about 12 weak and widely spaced denticles situated where the most distict spiral striae enter the lip; siphonal notch rather wide and very deep. Colour pattern consists of chestnut-brown squarish blotches on a white shell, the blotches being arranged rather regularly in 5 spiral rows on spire whorls and 15-16 rows on body whorl; most blotches extending over 2 to 3 spirals. Most of the spiral striae are underlined brown, expressed by brown points on the denticles of the outer lip.

The Nebularia thachi resembles Quasimitra cardinalis (Gmelin, 1791) , but is more slender in shape (width = 28-30% versus 35-38% of shell length), has a longer spire (46-51% versus 34-43% of shell length), has fewer and more widely spaced spiral striae (16-18 versus 28-29 spirals on body whorl), fewer and more widely spaced denticles (12 versus around 25) on the outer lip. Further differences are the rather smooth shell base in N. thachi versus the roughly sculptured shell base with raised threads in M. cardinalis. Finally the rather regularly arranged brown blotches in N. thachi are in contrast to the blotches usually fused into irregular axial streaks in M. cardinalis. Cernohorsky (1976a: 312-313, pi. 264) did not mention any record of M. cardinalis from Vietnam. Dr. Thach has collected miters along the coast of Central Vietnam (Provinces of Khanh Hoa and Ninh Thuan) since 1975 and found one specimen of the typical broad formed M. cardinalis in the Ninh Thuan Province of Central Vietnam. So it is evident that both species live sympatrically at the type locality of Nebularia thachi n. sp.
Nebularia thachi can not be confused with Quasimitra raphaeli Drivas & ]ay, 1990 from Reunion Island. The two types of the latter species are smaller (shell length 30.7 and 55.6 mm), their outer lip is thick and smooth, and the brown blotches are smaller, do not extend over several spirals but are restricted to individual flattened spiral cords; one peripheral cord does not bear any blotch.

Type locality:
Phan Rang City (11°34'N, 108°59'E, 80 km south of Nha Trang City), Prov¬ince Ninh Thuan, Vietnam.
Distribution and habitat:
Only known from Central Vietnam. Living in sand near reef at approx. 15 m depth.