Diagnosis.— Thin, encrusting mats of irregular shape, about 2-4 mm thick, occasionally with hillocks where the colony is considerably thickened, overgrowing dead, sometimes live, substrate. The large gastric cavities of the polyps extend right through the colony with only a thin layer of coenenchyme separating them from the substrate. Polyps are monomorphic, and totally retractile into apertures that may be slightly raised on calyx-like mounds. The coenenchyme contains large spindles, up to several mm long, ornamented with small spines. When very long, the spindles can be arranged as a conspicuous honeycomb-like network on the upper surface of the colony with a polyp within each ‘cell’. Smaller spindles may be grouped en chevron to form eight teeth surrounding each polyp aperture. The polyps contain similar spindles arranged as crown and points, and often have granular scales in the tentacles.
Colour.— Colony colour is variable. The encrusting mat can be greyish-purple, grey-blue, greenish-yellow, cream or pale brown, and the polyps intense greenishyellow, cream, orange, or greyish-purple.