Cantharidus opalus, common name the opal top shell or in the Māori language matangongore, is a species of large sea snail, a marine gastropod mollusc in the family Trochidae, the top snails.

These number about 7 on the penultimate whorl, mostly indistinct, crossed by more or less distinct oblique growth lines .

Its sides are straight or slightly concave, more or less eroded, and showing the iridescent green nacre at the tip.

The protoconch is conical, small, acute, and consists of 2½ convex smooth and pinkish-brown whorls.

[2] This marine species is endemic to New Zealand, where it is found subtidally in kelp beds down to at least 15m, on semi-exposed to exposed coasts.