Shell depressed, with 4 – 4,5 whorls; initial whorls strongly flattened and body-whorl slightly shouldered. Near the beginning of the body-whorl the periphery is angular to more regularly rounded and more or less clearly carinate; there is always a narrow crenulate band around the periphery, discernible as far as the peristome. Often the suture is partly situated slightly below the keel, which may be followed along the upper whorls then; otherwise the suture is not indented at all or it is situated in a slightly concave zone. Aperture more or less elliptical, sometimes with a rather prominent columellar angle below; the outer lip is clearly thickened inside. Umbilicus (nearly) completely closed by the reflected columellar lip.
The shells are dull. The colour pattern is extremely variable, with (dark) brown (interrupted) spiral lines and spirally arranged blotches on a whitish background. The aperture is whitish inside, sometimes with a pinkish hue at the parietal side; the outside pattern is shining through.
In juvenile shells the keel is (much) more prominent than in full-grown specimens.
Width 12.6-17.7 mm; height 7.7-12.0 mm.

T. impugnata can be distinguished from both T. geminata and T. grasseti by (1) the crenulate band marking the periphery of the body-whorl, which usually is sharply angular and (2) the dull aspect of the shell surface.
The other Theba species differ by these and additional characters from T. impugnata.
T. impugnata has been described and figured quite satisfactory by Mousson (1857: 133; 1872: 32, pl. 2 figs. 35, 36). The "var. subgeminata" has clearly been introduced for extreme forms of T. impugnata with a weekly developed peripheral keel, which becomes obsolete on the body-whorl. Such forms do not necessarily indicate hybridization with T geminata. They are as frequent as are very sharply keeled shells, which mark the other end of the range of variation concerning this character. The sculpture of the periphery of the shells is independent of the prominence of the angularity.
We could study eleven samples of T. impugnata from Lanzarote and found this species sympatric with T. geminata in eight of these. Where the two species occur together they are about equally common.

Lanzarote

The species is restricted to the northeastern part of Lanzarote and the adjacent islet of Graciosa.