The skeletal plates of the chitons, Katherina tunicata and Mopalia hindsii, are sturdy, constituting about one-fifth of the wet weight of the animals. They were seldom found broken in the specimens examined, but some broken plates were found undergoing repair. These chitons are seldom washed free from the substrate even during storms. The skeletal plates of the giant chiton, Cryptochiton stelleri, on the other hand, are flat and thin and constitute only 7.4% of the wet weight of the animal. Furthermore cryptochitons do not attach as firmly to the rocks or algae and are washed ashore in large numbers after a storm. The majority of cryptochitons examined showed breaks in one or more skeletal plates and in almost all of these, some degree of repair and deposition of membrane or mineral could be observed. The ability to repair its plates is probably of value to this species in view of the weakness in design of its skeleton.