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A medium-size light yellow-buff moth with a paler costa and reduced dark markings. Dark markings consist of a dark basal dash just below the cubital vein and black filling in the space before and after the rounded orbicular spot. The AM and PM lines are lightly marked, single and scalloped. The reniform is narrowly outlined and there are a number of small dark chevrons in the subterminal area. The hindwings are unevenly shaded with light and dark brown, often with a dark median line, and with the dark scaling more extensive in females. Male antennae markedly biserrate; female simple. Clausa is a member of the idahoensis complex, and is very similar to some of the others in the group. Lafontaine (1987) provides a key to the group, although the group is known to contain several additional undescribed species in Alberta. The genitalia of both sexes are indistinguishable from those of Euxoa idahoensis. Euxoa clausa appears to have a very restricted range. It was originally described from specimens from Lethbridge.

Clausa has a rather restricted range and is known mainly from the northwestern Great Plains in southern Saskatchewan and Alberta, south to southwestern Montana and Nebraska. In Alberta it has been found in the arid grasslands region, north to the Drumheller region. It was originally described from specimens from Lethbridge.