Shell : Elongate, fusiform, reaching about 51 mm length and 17 mm width Spire elevated, spire angle about 25°. Whole shell surface chalky, dirty white. Shell thin, fragile. Periostracum unknown. Protoconch dome-shaped, with about 5 mm maximum diameter. 2.5 whorls. Teleoconch with up to 4.0 convex whorls. First three teleoconch whorls with about 20 axial ribs. Remainder of teleoconch with axial sculpture of fine growth lines. Spiral sculpture of fine cords, about 20-25 in third teleoconch whorl, 80-100 in the last whorl. Suture excavated. Aperture elongate-elliptical (length/ width = 3.5). Outer lip with narrow varix. Parietal region well delimited, with indication of glazed surface (even in badly eroded type-material). Columella arched, with 2-3 columellar plaits. Holotype: MORG 25468, 38.3 mm length, 13.8 mm width, MD-55 station CP-01 (Beam trawl), off Rio de Janeiro State, Brazil. Type locality: Lower continental slope off the coast of Rio de Janeiro State, Brazil, 23°04'S, 40°20'W, at 2,370-2,380 m depth, muddy bottom.
Remarks: Tractolira tenebrosa is known only from dead shells collected at the upper abyssal zone (between 2,370 and 3,270 m depth). Allocation to subfamilial and generic level is here based solely on shell morphology and previous records of the other two congeneric species, T. sparta Dall, 1896 (Gulf of Panama) and T. germonae Harasewych, 1987 (South Sandwich Islands, Scotia Sea), from equivalent depths and bottom conditions. Tractolira tenebrosa differs conchologically from T. sparta and T. germonae by having a dome-shaped protoconch instead of the pointed, calcarella-bearing protoconch of the latter two species. Also, the first teleoconch whorls are more crowded, with smaller incremental angles in T. tenebrosa. Association of these two conditions produces a blunter, less conical profile in the early teleoconch whorls. The new species is shorter than T. sparta, has a proportionally larger aperture, and weaker axial ribs re-stricted to the two first teleoconch whorls. Axial ribs are absent in T. germonae; spiral threads are more numerous in T. tenebrosa (80-100, last whorl) than in T. germonae (60-80, last whorl), where they are also broader. The new species lacks the flared outer lip and the anteriorly twisted columella and anterior canal, observed in T. germonae.