Teratology is a congenital abnormality commonly observed in the shells of mollusks. A new case of teratology in polyplacophorans is documented. This corresponds to a new case of coalescence in the species Plaxiphora aurata, an extant marine mollusk from Punta Arenas (53°S), in southern Chile. In this specimen, the third and fourth shell plates were fused together, causing the fourth plate to be considerably reduced. To compare with another species of the genus several specimens of P. mercatoris, including a teratological specimen, were measured. Morphometrics suggest that chitons are similar in size and shape compared to normal individuals of the same species. Similar cases from the same region were also compared, where teratological individuals (with both hypomerism and coalescence) reached lengths of several centimeters, surviving for over several years.