 The thorax consists of 13 to 16 segments, with the axis wider than pleural regions, obliquely truncated or rounded pleural tips, and the fulcrum placed very close to the axis.The tailpiece (or pygidium) is small, with 3 to 5 axial rings, distinctly segmented pleural fields, and an entire, evenly rounded margin.The foremost pair of extremities in trilobites are the antennas.