Type locality: Seamount W. of Macclesfield Bank, South China Sea.

Description
Shell medium-sized, fusiform (W/H 0.37, A/H 0.47), with high spire, and moderately long, tapering siphonal canal. Protoconch pale, early whorls missing, last whorl evenly convex, glossy, unsculptured; protoconch-teleoconch transition clear-cut. Teleoconch of ca 9 1/3 whorls separated by canaliculated suture, giving spire stepped appearance. First three whorls subcylindrical, later whorls with increasingly more convex outline. Sculpture of strong, rounded ribs, intersected by spiral cords to form a somewhat beaded pattern, with deep pits in the interspaces between sculptural elements. Last whorl with 27 ribs, penultimate with 23. Subsutural band distinct on first five whorls, separated from whorl periphery by slight adapical depression on ribs. On later whorls subsutural band subdivided into three cords, becoming less distinct. Penultimate whorl with five cords, adapical three more closely set, corresponding to subsutural band. Last whorl with 10 cords on adapical part and periphery, followed by four gradually weakening whorls on shell base. Siphonal canal straight, with 8 strong continuous (not gemmate) cords, separated from shell base by a broadened interspace between cords. Aperture elongate, shallowly notched at tip of siphonal canal, with broad and low adapical callous pad. Outer lip convex in its adapical portion; constricted towards tip of siphonal canal below mid-height; inside with 8 strong lirae. Inner lip with five rather narrow and elevated columellar folds; uppermost strongest, subsequent gradually reducing in size.
Background color cream-white, with light orange spiral band over abapical spire cord, bordering lower suture on spire whorls. Inside of aperture white.
Radula of about 45 transversal rows of teeth. Rachidian with broad base, and central part elevated above radular membrane bearing three closely-set pointed cusps. Marginals, unicuspid, sickle-shaped.

Etymology
The species epithet refers to the well pronounced beaded sculptural pattern, and coronate adapical margins of late spire whorls.

Distribution: South China Sea, in 360-403 meters, and Coral Sea, Chesterfield plateau, in 568-570 meters.

RemarksThe single specimen from the Chesterfield Plateau is morphologically close to those from the type locality, however, it differs in having lower ribs, resulting in a more continuous subsutural band, colored orange on last whorl. The specimen retains an intact protoconch, which is high, cyrthoconoid, comprising ca. 3 glossy, evenly convex whorls lacking sculpture.
The species most similar to Tosapusia diadema sp. nov. is T sauternesensis (Guillot de Suduiraut, 1997) . The two species are similar in shell proportions, whorl profile, sculpture, and color pattern. However, Tosapusia diadema sp. nov. can be differentiated by a lower spire, coarser sculpture, with fewer cords per whorl, cord interspaces broader, and 'coronate' adapical whorls margins. Vexillena kurodai (see Fedosov et al. 2017: fig. 8 C, D [as Tosapusia]) is another somewhat similar species, with more flattened whorl outline, weaker cords lacking beads at intersection with axial ribs, so its sculpture looks more cancellate compared to that of T diadema.