Longchaeus achates is a species of sea snail, a marine gastropod mollusk in the family Pyramidellidae, the pyrams and their allies.

[2][3] The shell is stout, shining, milk-white, with irregular, flammulated spots of rust color, which are densest about one-third of the distance between the sutures anterior to the summits, where they form an almost continuous band.

The ten whorls of the teleoconch are weakly rounded, slightly shouldered and faintly crenulated at the summit.

They are marked by lines of growth, which are somewhat variable in strength, the strongest being on the side of the crenulations.

It is bounded by a low, strong basal fasciole, bearing three oblique folds, the posterior one of which is lamellar and situated a little anterior to the insertion of the columella; the other two folds are much less strongly developed and more oblique.