Umbonula megastoma (Busk, 1859)

Colonies are encrusting, multiserial and sheet-like, but occasionally bifoliate and multilamellar according to Lagaaij (1952, p. 92). The ancestrula and early astogeny have not been described.

Autozooids are rounded rhomboidal in outline shape, elongate, about 0.65 mm long by 0.40-0.50 mm wide. The frontal shield is moderately convex and variably granular, bordered by a large number of areolar pores which are radially elongated, and is produced into a low umbo over the suboral avicularium. Pseudopores are lacking. The primary orifice is hemielliptical, somewhat longitudinally elongate, large, about 0.28 mm long by 0.26 mm wide, the proximal edge being more or less straight. It occupies one-third to almost one-half of the frontal area of the zooid. Oral spines are lacking. Ovicells are apparently absent.

A small suboral adventitious avicularium is present in most zooids, inclined almost perpendicular to the plane of the autozooid and therefore often difficult to see in frontal aspect. This has a rounded subtriangular rostrum, directed proximally, and a calcified cross bar.