Well-hidden beneath the umbo of heterodont bivalves, a sack-like boring is etched into the two hinge plates of the opposed valves. The boring is abundant, occurs in numerous host species, ranges from Eocene to today and appears to have worldwide occurrence. The trace fossil is named Umbichnus inopinatus nov. igen. et isp. The nature of the tracemaker remains unknown. The possibility that the structure is a dissolution pit produced by the bivalve itself is discussed.