Now that I'm looking again, I think Mitrella cf. santabarbarensis (Gould &
Carpenter, 1857) would be a fair guess.

Keen's description: "The shell has more spiral grooves than most Mitrellas.
It is white, clouded or spotted with brown, with a row of spots below the
suture. About 8mm. The southern part of the Gulf of California to Central
America."

The chief differences are that mine lacks a subsutural pattern and its
whorls are straight-sided, whereas M. santabarbarensis has somewhat convex
whorls.

David Kirsh