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A medium-size (approximately 3.5-4.0 cm wingspan) moth with grey-black forewings and pale grey hindwings. The forewings are crossed with narrow, contrasting white "sawtoothed" basal, antemedian and postmedian lines. The reniform and orbicular spots are white, but filled or almost filled with dark scales and appearing as white rings. The subterminal and terminal lines are indicated by black scales forming small spots, and the fringe is lightly checkered, The hindwings are light grey, faintly banded and with a darker grey discal mark and narrow dark terminal line. Sexes similar and antennae simple. Very similar to but usually smaller than X. speciosa, which is usually lighter grey, not blackish, and has less contrasting white lines. There are also differences in the genitalia of both sexes (see Lafontaine 1998 for descriptions and images). In the mountains it is difficult to separate from dark forms of speciosa; in the boreal region it is most like X. perquiritata, which lacks the dark scales filling most of the reniform spot.

Adults are nocturnal and come to light. There is a single annual brood, with adults from mid July through mid August. The larva is described by Lafontaine (1998), and has been reared in the lab on blueberry (Vaccinium myrtilloides Michx.) (Ericaceae).

Bowman did not report mixta, possibly confusing it with speciosa. In the mountains X. speciosa occurs in several forms from grey to almost black, and some specimens of mixta can be difficult to identify correctly. The dark forms of speciosa can usually be recognized by the lack of the crisply contrasting white antemedian and postmedian bands.