A unique anomaly in the shape of the valves VII and VIII of a British Columbia (Canada) specimen of Tonicella lineata is described and figured. Valve VII has its lateral areas extremely extended posteriorly at the cost of valve III, of which only a triangular remnant is present. However, this shows normal apophyses; the abnormal valve VII has 4 slits on both sides instead of only 1. The anomaly may be due to disturbance at an early larval stage. Normally valves I-VII are formed by the fusion of three platelets, valve VIII only appearing later. Two of its three initial platelets might have fused with valve VII, resulting in valve VIII originating only from the central platelet.