These small, very flat crabs seldom exceed ½ inch in length. Their extreme compactness enables them to make use of small crevices not occupied by larger species. They have the ability to cast off voluntarily any of their appendages which might be grabbed by a predator or entrapped by a shifting rock. The missing appendage is usually regenerated in a short time. The carapace is nearly square and highly polished, giving this group of crabs their common name. The front legs and claws are greatly enlarged and flattened like the body. Dense mussel beds provide shelter for this crab and other intertidal invertebrates.