Characters: Colonies are upright, irregular, usually contorted, anastomosing bifacial plates. They are often over one metre across. Columellae are wall-like lobes. Colour: Deep bluish-grey or brown. Similar species: Pachyseris gemmae. See also P. involuta. Habitat: May develop into large mound-shaped colonies in shallow water but smaller colonies occur in a wide range of habitats including those exposed to strong wave action. Abundance: Common.