Cantharidus opalus opalus is a subspecies of large sea snail, a marine gastropod mollusc in the family Trochidae.
This subspecies is endemic
to New Zealand
including the Snares Islands
This top snail lives on coarse brown seaweed
at low water and below.
The shell is a large, tall-conical, with a broadly rounded periphery. The surface is almost smooth, except for subobsolete linear spiral grooves on the upper whorls
, and a few on the lower base, as well as faint axial lines of growth.
The external coloration of the shell grades from russet or orange on the early whorls, to mauve below, the whole crossed by closely spaced zigzag narrow axial stripes in darker mauve. The interior of the aperture is iridescent in a range of bright greenish, bluish and reddish colours.
The shell height is up to 43 mm, and the width is up to 28 mm.