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Genus Placocista

Shell is ovoid, flattened, with an acute lateral border; edge may bear many short, often paired, lanceolate spines, cemented between scales. Idiosomes are broad- or long-elliptic, sometimes circular, margins overlap. Aperture is large, narrowly biconvex, with pointed edges, in lateral view incised and surrounded by a thin rim of organic cement. Cytoplasm sometimes contains zoochlorellae. Habitat: mosses. Type species: Placocista spinosa (Carter, 1865)