Amastra subsoror was a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusc in the family Amastridae.

The whorls of the protoconch are reddish-brown, convex, and delicately striate, with arcuate striae that are thin and narrower than their intervals.

The subsequent whorls are strongly convex and brown, featuring subtle low growth wrinkles.

A white callus emerges, reinforcing the basal lip within, its inward portion visible through the shell as a yellowish or whitish band surrounding the axial crevice.

The columella is straight, its edge narrowly dilated and bearing a strongly oblique, relatively small lamella, which is more pronounced on the interior.