Mud mounds, described from the Volkhovian (earliest Middle Ordovician), east of St Petersburg (Dronov and Federov 1994), are 4 to 5 m high, 30 to 50 m across, dome-shaped, and with a mud core and micritic crust facies; no reported calcimicrobes. A Trypanites-bored discontinuity surface separates the micritic crust from flanking pelmatozoan bioclastic limestones. Pelmatozoans are thought by Dronov and Federov to have grown on the atoll-like micritic rim, as baffles to the inner lagoonal mud core. Palaeomagnetic drift paths (Torsvik et al 1992) suggest the mud mounds grew in cooler temperate shallow seas.