Fusinus undatus: Shell medium sized to large for genus (average 170.6 mm SL, n = 17). adults with 12-13 teleconch whorls, heavy and robust. Protoconch of two to 2.5 whorls: first whorl smooth, unsculptured, smaller in diameter than succeeding one: second whorl and onward bears fine axial ribs of varying strength. Maximum diameter approximately 1 mm. Early whorls bear thick, somewhat flattened spiral cords, lowermost abutting suture, increasing in number from four to six or seven by seventh whorl; thereafter, minor cords develop between majors, gradually thickening as teleoconch progresses. By 11th whorl, six major and numerous minor cords interspersed near periphery by numerous fine axial riblets that do not cross major cords. Axial riblets become fine fissures intersecting cords in area immediately below suture. Shell surface in later whorls distinctly glossy, with intersection of axial and spiral sculpture giving malleate appearance on final whorls. From approximately ninth whorl, spiral cord at periphery can become more prominent than others. Spiral cords become liner, more broadly spaced and less distinct in final three whorls: typically 11 cords above periphery on body whorl. Suture adpressed, no subsutural band. Axial teleoconch sculpture of heavy, rounded buttresses with straight to concave crowns, set oblique to axis with broadest span at upper extremity. Buttresses persist onto body whorl: those on early whorls often set in gaps between those on preceding whorl, giving appearance of axial torsion. On body whorl, previously subsumed lower angle of buttress becomes visible: lower angle rounded to sharp, but not bearing distinct cord. Profile of shell above periphery strongly concave; below lower peripheral angle on body whorl, profile sharply constricted with angular ramp connecting lower angle of buttress with upper section of neck. Neck thick and cylindriform in upper half to two thirds; lower portion tapering, often with strong recurvature near distal tip. Area from lower buttress angle to distal tip of neck bears numerous (typically approx. 35) indistinct spiral cords, irregularly spaced. Body whorl bears numerous very fine axial growth lines.
Aperture with thickened labral margin in adults that bears fine, sharp teeth formed at terminals of spiral cords on outer surface: labral margin gently sinuous, opisthocline in upper third, thereafter forming slight prosocline curve until constriction at entrance to siphonal canal. Parietal shield expressed with sharp edge from just below lower buttress angle to distal tip of siphonal canal. Parietal wall of aperture bears numerous fine discontinuous spiral threads that terminate some way behind parietal margin; at termination, threads may join each other or be broken into elongated papillae. Labral wall of aperture bears numerous (typically 30-35) fine spiral threads that terminate some distance from labral margin, leaving smooth band between their terminals and labral dentition; labral wall slightly rellexed just below suture terminus, forming channel. Labral margin of siphonal canal somewhat incurved in adults, constricting canal. Shell pale off-white to cream with occasional brown staining to upper spire. Primary whorls often with brown staining in interstices of axial ribs. Some specimens from Philippines bear single row of brown dashes between peripheral knobs from third or fourth to ninth or tenth whorl; knobs may also be lightly stained. Operculum typical for genus; thick and chitinous with nucleus at anterior terminus. Thin, pale brown periostracum on final whorls, rarely preserved in collections.

Fusinus undatus: Specimens from Japan are often more slender in appearance than those from the Philippines, with a correspondingly narrower parietal shield. The characteristic whorl profile and glossy surface are present in all cases, however. For further discussion see under E similis .

Fusinus undatus: Fusinus undatus is known from throughout the western Pacific, including Australia (Wilson. 1994) and the Coral Sea (Loch, 1986). Tahiti (ANSP), Belau (ANSP). Indonesia (Dharma. 2005) and Japan. In Hawaii it is replaced by the similar E mauiensis Callomon & Snyder. 2006. which can be distinguished by its smaller yet far heavier shell and characteristic twisted buttresses. A series of ten F. undatus specimens illustrated from a single locality in the Coral Sea by Loch (1986) approach F. mauiensis in some characters, particularly the shorter, thicker neck, but nevertheless can be distinguished by their more numerous buttresses with well-developed lower sections, and by their tall, slender spires.