Hippoporidra edax (Busk, 1859)

Colonies massive, multilayered as a result of extensive frontal budding of zooids, and always encrusting gastropod shells that were formerly occupied by hermit crabs (the hermit crabs themselves have never been found preserved in fossil examples). The original gastropod shell substrate is totally enveloped by bryozoan growth, resulting in a crudely gastropod-shaped colony with a large aperture. Lining this aperture, and forming the living chamber of the symbiotic hermit crab, is bryozoan interior wall calcification, rugose, porous and with lateral zooidal boundaries marked by salient thread-like walls. The colony surface is hummocky owing to a covering of monticules containing cortical zooids. Colonies typically have a chalky appearance and easily crumble, reflecting the aragonitic composition of the bryozoan skeleton.

Autozooids are small, less than 0.5 mm long, blister-like in vertical section, have a variable, rounded polygonal outline shape and are chaotically arranged. The frontal shield is granular, convex and has marginal areolar pores and scattered pores in more central locations. An umbo is usually developed immediately proximally of the orifice within a non-porous area of frontal shield. The small orifice measures 0.10-0.11 mm long by 0.07-0.08 mm wide, and is keyhole shaped with strong condyles dividing the slightly crescent-shaped poster from the larger, well-rounded anter. Overgrown autozooids often have orifices occluded by a closure plate with a groove representing the mould of the marginal opercular scerite. Oral spines are lacking or sparse. Ovicells (usually broken in fossils) are globular with a mostly uncalcified ectooecium and a flat entooecium containing an oval distal window.

Cortical zooids (presumed male zooids) resemble autozooids but are larger in overall size and have a slightly smaller orifice, 0.09-0.10 mm long by 0.07 mm wide.

Avicularia of two types - adventitious and interzooidal – are scattered among the autozooids between monticules. The adventitious avicularia are small, have a subangular rostrum and a calcified pivotal bar. Interzooidal avicularia are large, broad and rounded proximally, with a narrow, acute rostrum and a doubly constricted, ligulate pivotal bar.