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pygidium (adj. pygidial) The posterior part of the exoskeleton of a trilobite (Trilobita); it is generally formed by the fusion of several body segments but in some Cambrian forms consisting of a single segment. Many Cambrian trilobites have small pygidia and are said to be ‘micropygous’. Most trilobites are either isopygous, where the cephalon and pygidium are the same size, or heteropygous, where the pygidium is the smaller. In some cases the pygidium is larger than the head (macropygous).