In size they vary between wide limits, from minute sparkling points encrusting rock surfaces and often so thickly clustered together as to produce a drusy effect, to large single crystals measuring a yard in length and diameter and weighing half a ton.
Usually it is found on the British coast encrusting rocks exposed at low tides, or on the flat surfaces formed by sandbanks overlying clay, the latter kind of colonies being known locally as "scalps."
In the beginning of the 19th century worked ShOkwasai, who frequently collaborated with the metal-worker Shibayama, encrusting his lacquer with small decorations in metal by the latter.
In the encrusting type, which is found in a large proportion of the genera, the zooids are usually in a single layer, with their orifices facing away from the substratum; but in certain species the colony becomes multilaminar by the continued superposition of new zooids over the free surfaces of the older ones, whose orifices they naturally occlude.
In encrusting Ctenostomes and in the Membranipora-like Cheilostomes (figs.