Diagnosis.—Biogenic cavity characterized by an irregular, semicircular to circular outline largely positioned in the same plane of the substrate. Ends of the cavity may be bifurcated. Margin of the cavity may form lobes diverging outward. The cavity is subcircular to oval in cross−section.

Description.—The outline is highly variable and varies from horseshoe−like, i.e. semicircular, to a full circle (Figs. 1–3). Ends of the cavity may be bifurcated in some specimens (Figs. 1A, 2A). The ends are lobate or tubular. Apparently, there are no openings to the outer surface of the substrate. The margin is smooth where not covered by calcitic sparry cement (Fig. 4).