SHELL. Very large (up to 140 mm in length), teleoconch of up to eight whorls.
Protoconch of about three and one-half polished, conical volutions, terminating in sharp, crescentic varix.
Spiral ornamentation on first teleoconch whorl of two or three rounded cords, often masked by axial ribs. Spirals gradually increase in number until five alternating primary and secondary cords on third whorl. Tertiary threads intercalated until body whorl covered with complex pattern of three orders of magnitude: major cords at shoulder, periphery and base of whorl most prominent.
Axial ornamentation strong; on each of first two teleoconch whorls, nine rounded to weakly angulate ribs with small lamellar flanges looped into short open spinelel at shoulder and periphery of whorl. On third or fourth whorl every third rib strengthened to form small varix with short spine at shoulder and second, smaller, spine at periphery'; intervening ribs form elevated ridges lacking spines. On next whorl, an additional axial ridge added at adapertural end of intervarical area; intervarical ridges continue to increase in number but decrease in strength until body whorl; here six to eight rows ol nodules formed where very weak axials are crossed by spiral cords. Surface covered with numerous lamcllose growth lines, these, together with spiral threads, give shagrecned appearance. Three heavy, rounded varices per whorl, each with three long spines at crossing of primary cords. Shoulder spine longest, usually slightly recurved adaperturally. In many individuals, shoulder spine on dorsal (penultimate) varix extremely attenuated. Secondary spine anterior to each major spine shorter and slightly adaperturally curved. Small tertiary spinelcts randomly developed on some individuals. Siphonal canal with four or five relatively short major spines, second from base of body whorl longest, anterior two spines usually much shorter; very small adaperturally directed secondary spinelcts usually present.
Suture simple, crossed by frilled axial growth lamellae. Aperture ovate; margin of outer lip notched by extensions of median groove in spines, notches varying in size according to strength of spine. One internotch area on anterior third of outer lip projecting as a small labral tooth. Anal notch small. Inner lip slightly appressed posteriorly, not flaring, free-standing almost entire length, smooth.
Siphonal canal very long; terminations of previous canals fused into straight tube, open by narrow slit; spinose about three-fourths total length.
Colour ivory to golden brown, intervarical nodules lighter in colour; aperture white with reddish brown spots in and behind notches; interior white to light brown.

Heavy-shelled specimens of M. tenuirostrum are difficult to separate from some unicoloured specimens of M. aduncospinosus using teleoconch characters alone. The protoconch is the most reliable distinguishing character and the suture in M. aduncospinosus is channelled, being only indented in M. tenuirostrum. In addition, the axial ribs on the first two teleoconch whorls in M. tenuirostrum are simple and generally rounded, whereas in M. aduncospinosus they are usually subangulate. often with a rudimentary spine. This latter difference is not, however, consistent enough to be a reliable way to distinguish these taxa. The shell of Murex tribulus, by way of contrast, always has well-developed spines on all teleoconch whorls.
The syntypes of M. tenuirostrum lack a protoconch but the extremely long siphonal canal, and the teleoconch characters are typical of the species here associated with this name. In addition, the sculpture of the early whorls is also typical of this species.
The species has a wider distribution than M. tribulus, with which it is often confused, and the two taxa are frequently found living together. The shell of M. tenuirostrum can be separated from M. tribulus by its yellow brown rather than purplish colour, and fewer, much shorter spines on the anterior canal. Both of these taxa possess a conical protoconch suggesting that they have a planktotrophic larval life, and both are widely distributed.

Indian Ocean and west-pacific.