Costaclis mizon is a species of sea snail, a marine gastropod mollusk in the family Eulimidae.

[1] (Original description) The broadly subulate shell is high, conical, umbilicated, thin, glassy and feebly ribbed longitudinally.

The apex is small, quite regular, and perfectly rounded, with a minute flattening down of the extreme point of the first whorl, merely sufficient to prevent its being prominent.

The inner lip is entirely discontinuous across the body, and first makes its appearance in a small and slight porcellanous pad, which closely encircles the base of the columella.

The umbilicus lies behind the thin columellar lip and is a distinct, little, pervious, funnel-shaped pore, sharply defined by the intrabasal carination.