Shell small for genus (to 106.5 mm SL; average 91.3 mm SL, n = 7), thin and light. Protoconch of 1.5 whorls, smooth, glassy, inflated; final half whorl slightly constricted, with no varix at commencement of axial sculpture. Sculpture on early whorls of regular axial ribs that become angular by fourth or fifth whorl, crossed by six smooth spiral cords; interstices of cords with numerous pronounced axial growth marks and minor spiral lirae, giving clathrate appearance. Spiral cords become finer and more numerous in later whorls, with axial ribs gradually reduced to row of sharp, carinate shoulder knobs and faint axial buttresses on body whorl. Body whorl with five or more scattered spiral cords between shoulder and neck, three or more finer cords above shoulder. Suture plain in early whorls, slightly adpressed on body whorl; no subsutural band develops until final half whorl.
Labral profile of aperture straight and oblique above shoulder, rounded and occasionally slightly ventricose in lower part; slight constriction of aperture sometimes present at terminus of suture, corresponding to subsutural band. Labral wall of aperture with numerous fine spiral cords, not corresponding in position or spacing to external spiral sculpture; cords occasionally discontinuous or bifurcated; all cords terminate some way behind labral margin in adults. Labral margin of aperture thin, plain, with no developed dentition in adults. Parietal wall of aperture smooth; margin defined in adult specimens, but not detached from whorl face. Canal narrow, open: columella!' margin of canal not detached, labral margin thin.
Neck slender, strongly tapering, straight in upper part with mild to sharp curvature in lower third. Outer surface with numerous fine spiral cords that persist to distal tip.Shell often pure white overall, but some specimens (including holotype) pale to medium brown with irregular pale spiral bands; coloration becoming stronger on distal portion of neck. Aperture and canal always white.
Periostracum unknown; freshest of material to hand does not bear any trace of periostracum. Operculum unknown.

Fusinus penioniformis remains a rare species, with few examples known in museum or private collections. It is nevertheless sufficiently distinct from its sympatric congeners to be easily recognized. From the similarly-sized F akitai it can be distinguished by its more slender shell, angular whorl profile and row of carinate knobs on the shoulder. It differs from F perplexus by its smaller size, its angular profile and the absence of labral dentition in adults. Its angular profile invites comparison with F teretron, but the early whorls of F penioniformis are distinctly angular and both its axial and spiral sculpture are less developed.

This species is at present thought to have a very limited distribution. All the material examined that bears locality data comes from southern or southwestern Shikoku. and especially the area around Tosa-Okinoshima.