The shell is large (maximum length 165 mm) and fusiform to globose-fusiform. The spire is moderately low to moderately high, consisting of two nuclear whorls and seven or eight weakly shouldered postnuclear whorls. The suture is generally obscured by the succeeding whorl. The body whorl is large and globose to fusiform. The aperture is large and ovate, with a moderately deep and broad anal sulcus, this delimited parietally by a strong spiral ridge. The outer apertural lip is coarsely dentate and somewhat erect, most notably anteriorly. The columellar lip is adherent for its entire length, with a few weak or ephemeral pustules at its anterior end. The siphonal canal is generally broad, moderately short, narrowly open to the right, and weakly, distally recurved.
The body whorl bears six to eight varices, these sometimes spinose or nodose. Other axial sculpture consists of one to three ridges in each intervarical space. Spiral sculpture consists of alternating major and minor cords, these producing very short to moderately long projections where they cross the varices. There are generally two long, open or partially open projections (whether long spines or short, blunt nodes), the longer at the shoulder margin and a slightly shorter one just below it. A series of four shorter spines is followed by a gap and then by two short, open spines on the canal. Microsculpture, consisting of numerous fine, irregular spiral threads, crossed by fine axial lamellae, covers the entire shell.
Shell color is yellow-brown to pinkish-brown, with three spiral brown bands on the body, one at the shoulder margin, one medial, and one basal, these most noticeable on the varices and on the interior of the outer lip, where the color changes co pale pink or dark salmon-pink. The aperture is white within; the apertural margin is suffused with pale pinkish-orange or deep salmon.

In the form common to nineteenth-century collections, the spines are moderately long, broadly open, and straight to strongly recurved. This form, which grows to the listed maximum size, is the M. saxatilis of authors. Actually, Linne's concept of M. saxatilis is a composite of several species and is generally considered a dubious name. Another form, named Purpura rosarium Roding, 1798, has a much smaller shell (6.5-70 mm) and low, closed, nodelike shell projections. In this form almost all shell sculpture is reduced. To judge from the number of shell whorls present, the specimens to which this name is attached are generally immature.

Cape Verde Islands to Angola, western Africa.