Diagnosis: Shell fairly small (21 mm), b/1 0,30-0.33. a/I 0,31-0,36. whorls with a rounded shoulder, siphonal canal moderately short, tip obliquely truncate, with fairly deep notch; aperture narrow, fasciole fairly strong, labrum distinctly crenulate, stromboid notch well-developed, anal sinus openly U-shaped, directed slightly adapically, parietal nodule large, rounded, situated at suture; subsutural cord strong, weakly nodose, equal in width to moderately deep, concave shoulder sulcus; axial ribs strong, moderately opisthocline, terminating at sulcus, 11-12 on penultimate whorl, bearing strong, rounded to transverse nodules where crossed by spiral cords, which in rib intervals are somewhat tabulate and much wider than their interstices, 3-4 spirals below sulcus on penultimate whorl, sulcus with a shallow median furrow only; no intermediary or microspiral threads, growth-threads inconspicuous; 2-3 varices per whorl, terminal one mid-dorsal. Greyish-white with crests of ribs and a median zone paler, spirals tinged with yellowish-brown between ribs, varices followed by an axial orange-brown blotch, subsutural cord spotted with orange-brown. Protoconch rather papilliform. of nearly 2 whorls, breadth 0,73-0,78 mm.
Description: Shell narrowly claviform, b/1 0,30-0,33, a/1 0,31-0,36, with moder¬ately short, obliquely truncate base; suture moderately deep, teleoconch whorls about 10; each whorl moderately convex, periphery more or less median, slightly shouldered with a moderately deep shoulder sulcus (which contains a shallow median groove); subsutural cord moderately strong, angulate, crenate to subnodular, equal in width to sulcus or slightly wider; left side of base of body whorl
concave, fasciole moderately strong. Aperture narrowly pyriform, siphonal canal wide, expanded terminally, end quite deeply notched; labial callus thick, forming a strong, rounded nodule in posterior angle of aperture, not intruding into opening of anal sinus and hence scarcely constricting it. outer edge of callus only slightly free on columella; columella slightly convex, parietal region concave; labrum strongly convex in side-view, crenulated, with a wide, well-developed stromboid notch and deep, openly U-shaped anal sinus, which is directed slightly adapically.
Sculptured by strong axial ribs, crossed by weaker spiral lirae. which form low, rounded to transverse nodules on their crests; 2-3 varices per whorl from 3rd or 4th whorl on, these being strong and wide on later whorls, the last one being mid-dorsal; growth-lines fairly strong, sometimes forming low plicules on spiral ridges, particularly basally. Axial ribs more or less equal to intervals, opisthocline. straight on spire whorls, slightly arcuate on body whorl where they reach rostrum, evanescing abruptly above at mid-shoulder sulcus, crests of ribs angular in cross-section, sides steeply sloping; 8-9 ribs on 1st whorl, 11-12 on penultimate one (including varices), weak and irregular behind lip. Spiral lirae moderately wide and gently rounded (somewhat flatter between axials), with very narrow interstices, creating a sulcate appearance on later whorls; a total of 7-8 thin, close lirae on 1st whorl, 3-4 below shoulder on penultimate whorl. 14-16 on base of body whorl, those on rostrum narrow and rounded.
Colour [greyish-] white, spiral lirae marked between axials with light brown, each varix followed by an axial streak of moderate brown: crests of ribs, base and a median line around body whorl white, subsutural cord spotted with light to moderate brown, apical whorls dark brown; aperture and columella white.
Protoconch (Fig. 26) rather papilliform. of nearly two whorls, 1st one tilted; smooth except for growth-lines on last quarter-whorl; breadth 0,73-0,78 mm. height 0,68-0,80 mm (b/h 0,91-1,11).Dimensions: 20,3 x 6,4 mm (holotype); 21,0 x 6,5 mm, 18,9 x 5,7 mm (largest and smallest paratypc respectively).
Operculum oblanceolate with sharp terminal nucleus, translucent moderate yellow.
Body light yellow, siphon mottled with black, eyes large, tentacles short, only slightly exceeding eye-lobe in length; penis very large, dorsoventrally flattened, its tip conical, without a rectractile papilla, but opening of spermduct surrounded by a weak, crenulated flange. Radula small, of about 48 rows of marginal plates, which resemble those of I. arctatus

I. Isabella is closely allied to I. varicosus (Reeve, 1843) of the Philippines, and has been compared with three syntypes of that species and with a small NM series. In varicosus the shoulder is obsolete and the shoulder sulcus contains spiral threads instead of a shallow median groove; axial ribs are at most shallowly incised by the spiral sculpture, instead of being cut into weak nodules as in I. isabella; varices are wider and smoother in varicosus, its protoconch is less bulbous and the 1st teleoconch whorl is much flatter.

North of Mozambique