Shell of medium size, up to 33 mm length, fusiform, spire tall. Protoconch paucispiral, of 2.5 whorls, dome-shaped, about 450-500 µm in maximum diameter, smooth, color cream. Transition to teleoconch not well marked. Teleoconch elongate and lanceolate, with 8-9 whorls. Sculpture of weak, equally spaced, wavy collabral axial ribs, 10-11 on earlier whorls, 12-15 on last whorl, extending from suture to suture. Spiral lines 55-65, regularly spaced and of about same size, overriding axial ribs on last whorl. Suture well-defined. Aperture narrow and ovate, anterior and posterior sinuses not distinct, about one third of the entire height, inner color cream. Siphonal canal short and narrow. Columella curved, smooth. Outer lip thick, inner surface smooth. Shell color uniformly cream with brownish tinges. Soft parts unknown.

Horaiclavus sysoevi is herein described from four shells collected during the JME (1933-34) that were already reported and in part figured by Sysoev (1996: 2, figs. 20-21) under the name H. splendidus. We have been able to examine the holotype of H. splendidus from the H. Cuming collection and the four shells from JME. This has led us to unquestionably separate that lot of shells from H. splendidus. The new species is similar to H. splendidus, but clearly distinguishable by several diagnostic features. Horaiclavus sysoevi exhibits: more lanceolate and less cylindrical body shape*, bigger size, smaller and higher spired protoconch, more convex teleoconch whorls, small number of axial ribs, higher number of spirals, narrower and longer siphonal canal, different shell color. The new taxon is easily distinguishable from other members of the genus Horaiclavus: H. shitoensis Oyama, 1954 (p. 21, figs. 33-33a), Kuroda, Habe and Oyama (1971: 213, figs. 12-13), Oyama (1973: 50, fig. 7) and Tsuchida and Kurozomi (1996: 41, fig. 5 (1-3)) is much smaller (about 10 mm length), the teleo¬conch is less elongate and lanceolate, with only 5-6 whorls, stronger axial ribs and spiral sculpture. Horaiclavus sysoevi also differs markedly from H. filicinctus (Smith, 1882), this latter species being smaller (about 10 mm length), with a teleoconch more biconical and much less elongated, with only 5-6 whorls, stronger and fewer axial ribs, and the aperture about half of shell length, as can be observed in the pictures reported by Kuroda, Habe and Oyama (1971: 213, figs. 19) and Tsuchida and Kurozomi (1996: 41, fig. 8 (5)). These authors figured also another species of Horaiclavus that is somewhat similar in size and teleoconch shape to H. shitoensis but completely different from H. sysoevi. With this note the geographical distribution of Horaiclavus is expanded, now ranging from the Japan Sea to the northwestern Indian Ocean.

Gulf of Aden, northwestern Indian Ocean, 13°43'18" N, 47°56'4" E to 13°46'00" N, 47°50'42" E, 533 m, JME (1933-34), HEMS MABAHISS station 188.