In modern arthropods, the addition of new segments during development primitively occurs by budding anteriorly from an unsegmented terminal region. Trilobites also accreted segments by terminal addition. To determine the state of tagmosis and patterns of development of the zone of segment addition in trilobites, we re-examined the terminal region of the Ordovician pliomerid trilobite Hintzeia plicamarginis from the Broken Skull Formation, Mackenzie Mountains, Northwest Territories. The known pre-holsaspid ontogeny of H. plicamarginis consists of eight instars. Analysis using geometric morphometrics of the terminal piece and its anteriorly adjoining segment indicates that the terminal piece changes shape allometrically through the developmental sequence from the second through sixth instars but not thereafter; the most substantial change occurs between the fourth and fifth instars. In this molt, the terminal piece shifts from quadrangular to triangular in form. This change coincides with the cessation of terminal addition. The segments destined to become the mature pygidium, including the segment adjoining the terminal piece, become relatively longer and narrower through the end of meraspid development, but the shape of the terminal piece itself is constant from the sixth instar onward. Despite the modification of the final trunk segment in the fifth instar, such as the fusion of pleural spines posterior to the terminal piece, the continuous existence of an undifferentiated terminal piece from its segment-generating phase to the post-accretion phase indicates that the terminal piece is not itself a segment. Because a tagma must consist of two or more segments, there is no caudal tagma in H. plicamarginis. Trilobites, therefore, are primitively similar to other arthropod groups in having a distinct cranial tagma but modular trunk segmentation, from which additional tagmata can be secondarily derived in some groups, such as in scutelluids.