The thallus of
A. sordithecium is
crustose and grows on bark, forming a continuous patch up to about across. Its surface is smooth to uneven and olive-green to gray-green. In cross section, it has a thick, cartilage-like , a distinct positioned near the surface, and a distinct
medulla that is partly immersed in the outer bark and encrusted with numerous small gray crystals. The
perithecia (flask-shaped
fruiting bodies) are aggregated, with about four to eight grouped in rounded to slightly irregular, well-defined pseudostromata. The pseudostromata are about 1–2 mm across, prominent, and up to about 1 mm high, and are covered by a whitish thallus layer up to the shared
ostioles. The ostioles are fused and eccentric, forming one to three shared openings near the center. From above they appear as small black dots about 0.08–0.10 mm across and lie along narrow, triangular slits in the whitish covering that show as dark brown lines. Individual perithecia are pear-shaped, about 0.4–0.5 mm wide and up to about 0.6 mm high. The is and about 30–50
μm thick, and the spaces between perithecia are filled with amorphous orange-brown tissue. The hamathecium consists of densely interwoven, net-like
paraphyses embedded in a gelatinous matrix that is densely filled with dull yellowish-brown oil droplets (no
staining in
Lugol's iodine, or IKI−); the ostiolar channel is also inspersed.
Asci contain eight
hyaline, spindle-shaped ascospores that are three-septate, with and diamond-shaped , measuring about 30–40 × 10–12 μm (IKI−). The pseudostromata cortex contains lichexanthone and
fluoresces yellow under long-wavelength ultraviolet light (UV+ yellow), while lichexanthone is absent from the exposed thallus surface outside the pseudostromata. ==Habitat and distribution==