Amastra subrostrata was an extinct species of air-breathing land snail, a terrestrial pulmonate gastropod mollusc in the family Amastridae.
The length of the shell attains 15 mm, its diameter 8 mm
The shell is imperforate, dextral, ovate-conic, and solid, with an irregularly striate surface that is slightly glossy. Its coloration is brown, irregularly clouded with tawny hues.
The spire is swollen-conic with a rather acute apex and consists of six whorls. The first four whorls are barely convex, while the body whorl is rounded and accounts for about two-fifths of the total length. A nearly basal, acute, and oblique columellar lamella is present.
The aperture is slightly oblique and irregularly semi-elliptical, angular at the columella, giving it a beak-like extension. The peristome is unexpanded and white-lipped within, with the right margin slightly spreading and the columellar margin only slightly dilated and adnate.
This species was endemic to Hawai, occurring on Oahu.