Shell depressed, somewhat lens-shaped, with 4 – 4,5 flattened whorls; periphery strongly angular and sometimes slightly carinate near the beginning of the body-whorl, more rounded near the aperture. Umbilicus (nearly) completely closed by the reflected columellar lip. Aperture more or less elliptical; the columellar angle below may be rather prominent, i.e. more distinct than in the specimen figured. Usually the outher lip is not or only slightly thickened inside.
The shells are dull. All specimens studied have a varied pattern of (dark) brown (interrupted) spiral lines and spirally arranged blotches on a whitish background. The aperture is light brownish inside, with the outside pattern shining through.
Juvenile shells of c. three whorls have a more triangular aperture (apart from the incision by the penultimate whorl), with a prominent angle below and
at the periphery; the outer lip is more strongly developed than in full-grown specimens. Width 13.7-16.2 mm; height 8.4-11.4 mm.

T grasseti differs from T impugnata by (1) the periphery of the shell, which is angular with an obsolete keel at most, not crenulate, (2) the usually more simple peristome, without a thickened internal lip, (3) a slightly less prominent microsculpture, and (4) more fragile shell walls. The other Theba species can be distinguished from T grasseti more easily.

Range. — T. grasseti is only known from Gran Canaria. The species might be confined to La Isleta and the adjacent extreme northeastern part of Gran Canaria near Las Palmas.
Taylor (1912: 381) published interesting information on this species and its relation to T. pisana: "It . . . always lives apart from the typical pisana, being practically confined to the highlands and to the topmost ridges of the barrancos, and is never found naturally below an altitude of 400 feet, except on the Isleta, where the typical pisana does not exist. The var. grasseti is always found feeding on the Euphorbia balsamifera, while the typical pisana, which is exceedingly variable, swarms in the gardens, etc., feeding mostly on the Aloe (Agave americana) and the Prickly Pear (Opuntia dillenii) near the sea level, and is never found more than 100 feet above it.