Diloma subrostratum, common name the mudflat top shell, is a species of small sea snail, a marine gastropod mollusk in the family Trochidae, the top snails.

The solid, conical shell is suborbicular.

It is often more or less black, with close wavy longitudinal yellow lines.

The last whorl is large, rounded, with its hinder part with three to six spiral keels.

[2] This common estuarine species is endemic to New Zealand, occurring in the North and South Islands, and in Stewart Island.