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Pencilletta penicillata (Fabricius, 1780)
Colonies comprise an encrusting base of bifurcating branches from which arise one or more small fungiform (mushroom-shaped) subcolonies, these being the parts recognizable in fossil material from the Coralline Crag. Each fungiform subcolony has a narrow stalk formed of a bundle of erect autozooids and covered by their exterior wall calcification. The short stalk expands into a head with a flat or concave upper surface on which open the autozooids and gonozooids, the former arranged initially in uniserial but later in biserial fascicles radiating from the centre of the subcolony. New fascicles are intercalated towards the marginal growing edge. Small zooidal buds are visible on the basal side of the growing edge, forming a budding lamina that has its origin in the expanded exterior wall covering the stalk of the subcolony.
Autozooids are fixed-walled and have connate, longitudinally ovoidal apertures about 0.16 mm long by 0.12 mm wide. Peristomes and frontal walls contain sparse, longitudinally elongate pseudopores.
Gonozooids occupy the centres of the subcolonies, with lobes extending outwards between the autozooidal fascicles. The roof of the gonozooid is bulbous and the apparent ooeciopore, located roughly midway between the centre and outer edge of the subcolony, is adnate to an autozooidal peristome and similar in diameter to the aperture of an autozooid.