The fact that teloblastic somites form at the posterior end of the pygidium is fundamental to Stormer's theory that thoracic pleurae of trilobites consist of fused portions of 2 successive somites, with the pleural furrows as the somitic boundaries. Several lines of evidence are presented in support of this theory. Pleural furrows are consistently well developed throughout adult and larval pygidia, while commonly interpleural furrows are present only anteriorly. Somitic anomalies on the pygidium are bounded by pleural furrows. Secondary segmentation is commonly present in living arthropods and is the probable condition of the trilobite axis. Features of the adult and larval cephalon indicate its posterior edge is intrasomitic. Evidence opposed to this theory is reviewed.