Found at the south end rocky spit of Constellation Marine Park 4/27/13 IMAG2462 Flattop crab. Distinguished from flat porcelain crab (Petrolisthes cinctipes) by the blue (vs red) spot on the internal joint of the pincers.
Porcelain crabs are decapod crustaceans in the family Porcellanidae, which superficially resemble true crabs. They are a good example of carcinisation, whereby a non-crab-like animal (in this case a relative of a squat lobster) evolves into an animal that only a specialist would know is not a true crab. They live in all the world's oceans, except the Arctic Ocean and the Antarctic.