Very similar to M. pecten pecten but differs in morphology and colour of protoconch and details of sculpture and colour of teleoconch. Protoconch of two to two and one-quarter whorls; first whorl bulbous, terminated by prominent, flanged varix. Teleoconch as in M. pecten pecten except for following characters.
Spiral ornamentation of 3-4 prominent cords on first teleoconch whorl; on later whorls, consisting of primary, secondary and tertiary spirals; quaternary spirals extremely weak. Spines as in typical subspecies except secondary and tertiary spines relatively shorter than in most specimens of M. pecten pecten, particularly on anterior canal. In most specimens shoulder spines less adapically directed than in M. pecten pecten. Axial ornamentation differs in markedly more persistent intervarical nodes, these persisting until beginning of body whorl in largest specimens. Low axial growth lamellae prominent over whole surface but not as raised in interspaces between spiral cords as in typical subspecies.
Colour of protoconch brown. Teleoconch pale orange-brown background, often with diffuse white bands below suture, on periphery and on upper base, resulting in background colour remaining as brown bands in most specimens. Axial ribs and Varices, as well as three diffuse spirals crossing them, tend to be white to dirty white. Spirals between axials usually brown, often darker than interspaces, resulting in spotted pattern in some specimens. Spines dirty white with pale brown tinge. Aperture white, usually with brown spots in grooves in outer lip.
Remarks. The shells of Western Australian specimens of M. pecten are consistent in the differences described above from specimens from other parts of the Indo-West Pacific. The brown, bulbous protoconch, the more strongly developed axial ribs and the development of colour bands are the most distinctive features of the new subspecies.