Thallus effuse, rimose to rimose areolate, forming angular areoles c. 0.15–0.5 mm wide, whitish to pale buff, matt, sometimes with ferruginous speckling when on iron-rich rock. In section: to 150 µm thick, ecorticate but with a 7–10 µm thick, hyaline epinecral layer; algal layer c. 50–75 µm thick; medulla hyaline, but obscured by numerous mineral fragments, I–. Photobiont cells globose, 7–12 µm diam. or ellipsoid to 12 × 9.5 µm.
Apothecia mostly sessile and ±constricted below, marginate with ±plain disc, 0.2–0.55 mm diam., sometimes becoming convex with a reflexed margin to 1 mm diam., black (even when wet), epruinose; margin c. 0.05–0.06 mm wide. Exciple 50–60 µm wide, densely pigmented purple-brown and not clearly delimited from the hypothecium, K± purple intensifying, N+ red; hyphae c. 3.5–5 µm wide, short-celled, 5–7 (–10) µm long, giving a ‘scleroparenchymatous’ appearance to the upper and outer exciple in thin sections. Hymenium 45–52 µm tall, hyaline or with grey-brown to dark green vertical streaks, I–; epithecium not or only poorly differentiated. Hypothecium mostly concolorous with exciple, combined height of hypothecium and exciple c. 110–210 µm; uppermost part of hypothecium sometimes dark green, K–, N+ red. Paraphyses simple or occasionally once-forked, c. 1.5–2 µm in mid-hymenium; apices incrassate to clavate, 3–4 µm wide, hyaline or smokey grey-brown; in vertical streaks paraphyses are entirely surrounded by grey-brown or green pigment. Asci clavate, 45–50 × (12–)15–20 µm, 8-spored, ascospores coiled; in K/I outer coat and tholus amyloid, but with a non-amyloid, triangular axial body. Ascospores hyaline, curved to mostly sigmoid, (15–)16–24 × 3–3.5 µm, 3-septate, without a perispore.
Pycnidia numerous, immersed, black, 50–80 µm diam.; wall purple-brown, K–. Conidiogenous cells slender, 11–15 × 1–1.3 µm. Conidia curved or hamate, 14–19 × c. 0.7 µm.
Chemistry. Thallus K–, C–, KC–, P–, UV+ pale yellow; zeorin by TLC.
Distribution and ecology: Known from only two localities on Campbell Island, where it grows at low altitudes on coastal rocks. Most of the collections are on non-calcareous rock, but the rock in Harris 5363 is weakly calcareous in part. Associated species on the actual collections include ?Lecania sp., Placopsis sp., Verrucaria sp., whereas other collections from the same localities include Buellia sp., Caloplaca sp., Carbonea phaeostoma, Catillaria glaucogrisea, Lecanora capistrata, and Pertusaria graphica.
Notes: Microscopically, this species is readily identified by its mostly sigmoidally curved ascospores. We have been unable to find a comparable spore description in any other Bacidia species. Similar spores are found in some species of Scoliciosporum A. Massal., but that genus is characterized by a different exciple stucture, and by having branched paraphyses and a Lecanora-type ascus. However, B. curvispora is apparently close to B. gallowayi (below).
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