Life habit: non-lichenized, lichenicolous Thallus: absent Apothecia: round, immersed in basally constricted galls, dispersed, single or in groups of 2-5, 0.2-1 mm in diam. disc: black, flat, covered by a dense, white pruina exciple: hyaline, frequently pale brown in the upper part, thin, 12-20 Ám thick, composed of textura intricata, hardly distinguishable from the hypothecium below, I+ and K/I+ blue, K- epihymenium: pale brown, covered by a dense layer of reniform to discoid crystals not dissolving in K, I+ blue, K- hymenium: hyaline, not inspersed with oil droplets, 85-105 Ám tall; hymenial gel: I+ dark red, K/I+ blue; paraphysoids: branched, 2-2.5 Ám, apically slightly enlarged, 3-3.5 Ám, not distinctly pigmented; hypothecium: hyaline to very pale brown, 30-230 Ám thick, particularly thick under the center of the ascomata, I+ and K/I+ blue, K- asci: narrowly clavate, often with a long and narrow base, 65-85 x 12-14 Ám, (4-)6(-8)-spored; wall: apically thickened, K/I-, except for a K/I+ blue apical ring ascospores: hyaline, 3-septate (not or slightly constricted at septa, the two middle cells squared, the end cells elongate triangular), fusiform, 18-24 x 5-5.5 Ám; perispore: indistinct, brownish pigmentation not observed Pycnidia: immersed in hymenium; wall: hyaline below, pale brown near the ostiole conidia: hyaline, bacilliform, simple, 5-6 x 1 Ám. Host: on the upper surface, frequently at the margins of the thallus of a terricolous Physconia species close to P. muscigena; gall-inducing, but otherwise not visibly damaging the host World and Sonoran distribution: known only from the type locality in Guadalupe Island (Baja California). Notes: This is the first known non-lichenized, lichenicolous species of Schismatomma. It should not be confused with Opegrapha rotunda Hafellner, another lichenicolous Roccellaceae confined to Physconia thalli; that species is similar to Schismatomma physconiicola in its round, black, pruinose ascomata with 3-septate ascospores and the selection of Physconia as the host genus, but differs in having sessile ascomata with a strongly constricted base and a black exciple; it is not known from America.