Small round holes, Oichnus Bromley, are a locally common feature of crinoid pluricolumnals in the Mississippian of the British Isles. Numerous examples have been found from mudrocks in the Brigantian (Mississippian) Blackhall Limestone, Lower Limestone Formation, at Trearne Quarry, near Beith, north Ayrshire, all assigned to Oichnus simplex Bromley. These trace fossils are typically associated with growth deformities of the pluricolumnals, which are commonly swollen and more rarely grew a lip around the pit. Oichnus simplex is commonly centred on a columnal latus and adjacent sutures between columnals are deflected towards it. More rarely, pits are centred on the sutures between columnals. The O. simplex borings are interpreted as domiciles developed in live crinoids by an indeterminate, unmineralized invertebrate. The pluricolumnals are similar and are presumed to be derived from a single species, perhaps the poteriocrinine cladid Ureocrinus bockshii (Geinitz), the only nominal crinoid recorded from this site.