TYPE MATERIAL - Holotype (lv) NMNZ M.274915 (h 90.1 mm) and paratype both NMNZ M.274915. TYPE LOCALITY - New Zealand, Kermadec Ridge, S-SW of Tonga, Monowai Caldera, 25°48' S, 177°10' W, 1026 m [NIWA 32231: stn 2007204]. MATERIAL EXAMINED-Known only from the type material. DISTRIBUTION-Known only from the type locality. The species lives on the border of a semi-active caldera. DESCRIPTION - Shell large, up to 90.1 mm, rather thin but solid. Colour pale brown to grey where visible through periostracum. Shape broad, fusiform, spire high, siphonal canal short, wide, open. Apex eroded, 4.25 teleoconch whorls remaining. Whorls convex. Subsutural concavity narrow, shallow, on penultimate and last whorls only. Spiral sculpture of fine spiral grooves; roughly eight traces of spiral lines appearing on last part of penultimate whorl of holotype, seven fine, obscure spiral grooves on penultimate whorl of paratype. Interspaces wide, of equal width. Last whorl of holotype with 18-19 spiral grooves, of paratype with 30 grooves, interspaces on whorl side wide, of equal size; spiral grooves wider and deeper on base, with narrower interspaces. Axial sculpture present on upper whorls only, eroded on holotype; 19 axial ribs on first intact whorl of paratype, on subsutural slope, abapical portion of whorl smooth. Axial ribs narrow, pronounced, with constricted appearance; interspaces roughly twice as wide as one rib. Aperture dark brown inside, outer lip with white axial bands, purplish brown near lip; columella white, gently curved, with smooth, glossy callus; adapically with broad, flat columellar knob. Aperture plus siphonal canal less than half of total shell height. Periostracum greenish (paratype) to dark brown (holotype), rather thin, strongly adherent; covered with fine incremental lamellae when fresh (subadult paratype), polished on dorsal side of adult holotype. Operculum rather thick, corneous, pale brown (paratype) to dark brown (holotype); slightly smaller than aperture; semi-oval, nucleus terminal. REMARKS - Enigmaticolus marshalli n. sp. is characterized by its large size, the subsutural concavity, the narrow axial ribs on the subsutural slope of the upper spire whorls, and the spiral grooves across the entire surface. Thermosipho desbruyeresi (Okutani & Ohta, 1993) differs from E. marshalli n. sp. in its shape, with a more laterally flattened spire, the absence of a subsutural concavity, the smoother surface, the typically brown periostracum, and the smaller adult size. Enigmaticolus voluptarius n. sp. differs from E. marshalli n. sp. in its heavier shell, the broader shape with more constricted base, the deeper subsutural concavity, the wide and more oval aperture, and the absence of spiral grooves on the whorl sides. ETYMOLOGY - Enigmaticolus marshalli n. sp. is named in honour of Bruce Marshall, Museum of New Zealand Te Papa Tongarewa, for his important contributions to malacology and for his friendship.