A refining plate segment for a mechanical refiner of lignocellulosic material including: a convex conical refining surface on a convex conical substrate of the plate, wherein the refining surface is adapted to face a concave conical refining surface of an opposing refiner plate, the convex conical refining surface including bars and grooves formed between adjacent bars, wherein an angle of each bar with respect to a reference line parallel to a rotational axis of the refiner increases at least 15 degrees and the angle is a holdback angle is 10 to 45 degrees at a periphery of the refining surface, and wherein the bars each include a leading sidewall having an irregular surface having protrusions extending outwardly from the sidewall toward a sidewall on an adjacent bar.