 Segments in the pygidium are similar to the thoracic segments (bearing biramous limbs) but are not articulated.Trilobites can be described based on the pygidium being micropygous (pygidium smaller than cephalon), subisopygous (pygidium sub equal to cephalon), isopygous (pygidium equal in size to cephalon), or macropygous (pygidium larger than cephalon).Trilobite exoskeletons show a variety of small-scale structures collectively called prosopon.