Description: Shell elongate ovo-conical, relatively thin, lustrous, translucent white. Protoconch
of 1.5 whorls, diameter 260 µm, translucent, smooth; protoconch heterostrophic at angle of ca. 120° to teleoconch axis; 30-35 % of protoconch subsumed in first teleoconch whorl, with a half of earliest portion exposed Teleoconch of 3.3 whorls, convex-sided. Suture well constricted. Growth lines fine, orthocline. Teleoconch sculpture of distinct axial costae; costae slightly flexuous, ca. 22 in number on body whorl. Intercostae slightly wider than costae, with three narrow, distinct spiral ridges at nearly equal intervals. Suprasutural part and periphery of body whorl encircled by an additional spiral ridge, at which axial costae terminate and connect to each other. Body whorl 58 % of shell height; periphery convex. Base with faint, feeble axial costae. Aperture 38 % of shell height, ovate but slightly quadrate. Columella thin, slightly twisted at adapical part, forming an indistinct, feeble columellar fold. Inner lip covered with a thin callus. Outer lip thin. Palatal sculpture absent. Umbilicus not open.
Type locality: Kiyogahama Beach, Abu-cho, Abu-gun, Yamaguchi Prefecture.
Etymology: Truncatella (Latin): a genus of the family Truncatellidae; formis (Latin) shape. The present species is named for its similarity with young shells of Truncatella.
Remarks: Trabecula truncatelliformis is similar to a European species, Trabecula eximia (Jeffreys, 1849) (e.g. van Aartsen, 1977: p. 51, pl. 1, fig. 5), but differs from the latter in having less convex-sided whorls and in having a part of the earliest portion of the protoconch clearly exposed.
The present species is placed in the genus Trabecula, by virture of its having an ovo-conical shell with constricted sutures, distinct axial costae and slightly folded columella, all features of the type species Trabecula jeffreysiana Monterosato, 1884.