Fusinus similis: Shell medium size for genus (162.2 mm SL, n = 12), elongate fusiform, teleoconch of 10-13 whorls (average 11.5, n = 10). Protoconch damaged or lost in all material examined; from remains, estimated approximately 1.5 mm in diameter. Early whorls usually eroded, but bearing thick, prominent spiral cords, lowermost abutting suture, increasing in number from four to six or seven by seventh whorl; thereafter, minor cords develop between majors, tending to thicken as teleoconch progresses but varying in prominence. Spaces between spiral ribs with numerous axial growth lines whose interstices can be deep enough to form decussate sculpture in early whorls. Spiral cords remain dominant sculptural feature onto body whorl. Axial sculpture in early whorls of thick rounded ribs, contiguous but slightly staggered in vertical placement; major spiral cords cross ribs without weakening. Ribs shorten to become knobs after 5th to 7th whorl; knobs persist onto body whorl, occasionally weakening in final quarter turn. Suture adpressed, smoothly but irregularly undulating, no subsutural band. Lower angle of body whorl normally rounded, but occasionally slightly angulate; however, knobs do not form buttresses. Profile of shell above periphery straight to concave, creating distinctive angularity of upper aperture. Neck slender and tapering, straight or weakly recurved near distal tip. Spiral cords persist to tip of neck, their angle relative to axis steepening distally.
Aperture normally angulate in upper part, with little or no thickening of labral margin in adults. Adult labral margin bears prominent, sharp teeth formed at terminals of spiral cords on inner surface; viewed laterally, labral margin parallel to axis. Parietal margin of aperture slightly expressed in some adult specimens to form long, narrow shield that enters canal: parietal margin of canal only occasionally expressed, and then only in distal quarter. Parietal wall of aperture normally smooth, though occasionally with two indistinct spiral cords directly adjacent to suture. Labral wall of aperture bears numerous distinct spiral threads that terminate close to labral margin, leaving narrow and often indistinct smooth band between their terminals and labral dentition. Labral wall slightly reflexed just below suture terminus, forming channel. Canal usually open and unconstricted over entire length, even where neck is recurved. Operculum typical for genus; thick and chitinous with nucleus at anterior terminus. Thick, medium brown periostracum on entire shell, especially in Japanese specimens.

Fusinus similis: Fusinus undatus can be distinguished from F. similis by its generally heavier construction, its less recurved neck with a distinctly thickened upper portion, and by the persistence of the axial sculpture in proportionate scale to the end of the body whorl, which results in a relatively smaller aperture. The body whorl of F undatus bears heavy angular axial buttresses, whereas the sculpture in F. similis is normally a row of knobs at the shoulder, with little or no lower angulation of the body whorl. The surface of the later whorls in some F undatus specimens and the body whorl in all of them is distinctively glossy. The aperture of F. similis is usually angular in its upper part and has a less thickened margin in adults, with finer dentition. The parietal shield is longer and narrower. It is easy to separate the two species when only their typical forms are considered, but as with so many other Fusinus there are a number of variants of both that share some major characters and might at first glance be considered intergrades. A good example is one of the two specimens examined here from Sesoko, Okinawa (Fig. 160) and here assigned to F. undatus for its angular body whorl profile and early sculpture of fewer and larger knobs. Its slender neck, thin shell and light weight nevertheless initially led the authors to identify this subadult specimen as F. similis. Although the lack of typical fusinine spiral cords on the body whorl in most F. undatus has been noted from the beginning, some of the Japanese specimens examined here bear distinct spiral sculpture, as do some that are figured in literature. One example was figured by Kira (1954: pl. 29, fig. 5), and represents a form of F undatus that has since been cited as F undatus similis in the Japanese literature to distinguish it from the nominate form. Though it bears distinct spiral cords, it differs from F. similis in having a shorter, thicker neck that is not recurved, and in the persistence of the axial buttresses onto the body whorl. Another problematic figure is in Abbott & Dance (1982: 187, center row, second from right; as F undatus). It shows a pair of specimens that apparently represent F. similis. The shell on the left is unusual in that it retains the peripheral knobs on the entire body whorl, but its slender neck and rounded lower body whorl distinguish it as F. similis. The absence of the original material or the locality information for these specimens makes further investigation impractical. One further notable difference between F undatus and F similis is the very broad range of adult size in the former.
Fusinus galatheae Powell, 1967 (Callomon & Snyder, 2006: figs 28. 29) was compared in the original description with F colus and F nicobaricus, but not with F similis, which it resembles considerably more than either of the former. It may nevertheless be distinguished from F. similis by its thinner construction, more angular spire whorls, and the single row of regular knobs that become reduced only on the final part of the body whorl. In addition, the parietal shield in F galatheae can be strongly expressed and thickened, a feature it shares with F. bounty Rehder & Wilson, 1975 (Callomon & Snyder, 2006: figs 30, 31), a species so far known only from Pitcairn Island in the southern Pacific. F. galatheae is smaller and thicker than F. bountyi, with fewer and more rounded knobs at the shoulder. There are some similarities between F bountyi and F. similis. particularly in the angular aperture profile, the rounded lower body whorl and the persistence of spiral cords over the whole shell, but F bountyi can be distinguished by its more pointed shoulder knobs and resulting angular whorl profile, as well as by the highly expressed and thickened adult parietal shield.

Fusinus similis: The known range of F. similis falls within that of F. undatus, in the same way as F. ferrugineus is found within the range of the morphologically similar F. perplexus (see Callomon & Snyder, 2004). West-Pacific to New Caledonia.