or Ligia oceanica, a woodlouse that has evolved to make its living on the shore.
This one had wedged itself into a crevice about four feet up a mudstone cliff, waiting out daylight; slaters are nocturnal and will eat anything they wander across (but are apparently fond of Fucus vesiculosus, bladder wrack). If picked up by a young field assistant their sole means of defence seems to be a panic pooh. This is only the second slater I have seen in many years of intermittent beach naturalizing. This one was just shy of 2cm in length.
Latin and linnean stuff: the slater is an oniscid isopod belonging to the wonderfully-named class malacostraca.