Shell very large for genus (to 337 mm SL; average 291 mm SL, n = 12 adults), robust but light. Protoconch unknown, but probably very small, with final whorl diameter approximately 1 mm (see remarks). Teleoconch of 14 whorls; whorl profile initially rounded, becoming somewhat angular in later growth stage. First six or seven whorls bear heavy, rounded axial ribs, spaced randomly relative to those on following whorl; ribs then becoming more angular and reduced to series of shoulder knobs by penultimate whorl. Early whorls bear five strong major spiral cords with single faint minor cords in interspaces; minor cords strengthen in later whorls, becoming equal in strength to majors In sixth or seventh whorl. Two or three cords that cross axial knobs al periphery then become stronger than others, with single minor between them in final whorls; knobs can thus be strongly carinate (three cords, strongest at periphery) or bluntly angular (two cords, separated by periphery). Cords extend over half way down neck, becoming fainter or absent at distal tip: eight to nine majors above periphery on body whorl, with occasional minors, 40-45 cords below periphery, with scattered minors. Suture adpressed, with distinct subsutural band bearing several finer spiral cords. Numerous very fine axial growth marks over entire teleoconch. not crossing spiral cords in early whorls; later whorls bear scattered axial growth ridges not coinciding with shoulder knobs; ridges heavy enough to bend spiral cords. Neck long, smoothly tapering with only very gentle recurvature. Aperture ovate; labral margin forms gentle angle slightly above position of shoulder knobs; second, lower angle forms in mature adults due to ventricose flaring of lip. Interior of labral wall with numerous strong spiral cords, corresponding to troughs between major spiral cords on outer surface; inner cords remain strong to margin, terminating as strong denticles in adults. Columellar margin expressed, slightly flaring in adults to form parietal shield in lower two thirds: shield thick with sharp edge, bearing numerous short spiral cords shortly before margin; cords wavy, sometimes bifurcate or prematurely terminated, longer at posterior end of aperture where they extend far into shell. Inner margin of canal detached over entire length, continuous from parietal shield. Outer margin sharp. Canal broadly open over entire length. Shell dull off-white overall; later major spiral cords pale brown in some specimens.
Operculum typical for genus; thick, dark brown, chitinous, with nucleus at pointed tip. Periostracum: thick, yellow-brown, eroded from first six or seven whorls, bearing extremely fine axial lamellae with numerous curved and pointed thorns; thorns present over whole shell, becoming slightly larger towards top of subsutural band.

Murex longissimus Gmelin. 1791 was proposed based on the specimen figure by Chemnitz (1780: pl. 145. fig. 1344). Chemnitz also figured an example of this species in the previous plate (1780: pl. 144, fig. 1339). stating that he could see no difference between the two. He reproduced some notes from Martini that did not make any distinction either. Gmelin. however, named this earlier figure Murex candidus. Neither Bosc (1801) nor Dillwyn (1817) synonymized the two; Dillwyn treated them both as varieties of F colus (Linnaeus, 1758). The first reviser was Lamarck (1822: 122), who used Fusus longissimus and simultaneously cited F. candidus. Roding (1798: 121, no. 1562) had meanwhile introduced the name Syrinx producta for Chemnitz's fig. 1339, but cited Gmelin's F candidus too. making F. producta an objective junior synonym of F. candidus.
F. longissimus is rare in Japan, the examples cited here being the only ones so far seen by the authors. Though long known locally, it was first formally recorded only recently (Kimura. 1997). In adult form it is easily distinguished from other Fusinus by its enormous size, thin shell and carinate or blunt shoulder knobs. The only species in Japan that approaches it in size is F salisburyi, but the latter can be easily distinguished by its thicker shell with highly inflated whorls, its strong and complex spiral sculpture and its well-developed parietal shield with a detached and reflexed margin at the entrance to the canal. Fusinus colus can occasionally approach F. longissimus in size, but is more slender, with rounded whorls and a longer, more strongly recurved neck .
No significant difference is apparent in the thickness or composition of the periostracum between Japanese and Philippine examples of F. longissimus, though its color varies somewhat. The protoconch was not present in any of the material examined, but some specimen have a complete spire up to the separation point, and it is thus possible to estimate the size of the larval shell as between 1 and 2 mm in maximum diameter, which is consistent with other large Fusinus species.
The shell cited and figured as F. longissimus by Kiener and copied by Tryon (1881: 56, pl. 34, fig. 120) is apparently not this species; it is probably a striate specimen of F undatus (Gmelin, 1791). the only other species that grows that large (though not in Japan) and that has axial buttresses on the body whorl.

In Japan. F. longissimus has been recorded from off the Ogasawara Islands (Kimura. 1997). the Kii Peninsula in central Honshu (Callomon & Snyder, 2007) and off Iejima, Okinawa (Kimura. 1997). Outside Japan. F. longissimus is known from the Philippines. Taiwan. Vietnam and New Caledonia. It is taken on sandy substrates at between 50 and 120 m, and is apparently scarce throughout its range.