Hinnites giganteus (Gray, 1825)

The Giant Rock Scallop is a most unusual species and shows characteristics somewhere in between Spondylus and Pecten, for it starts life growing as a free floating bivalve (which is still visible in this picture if you look closely) but later when maturing it attaches itself to a rock and grows like any oyster or Spondylus with the lower valve firmly attachet to the underground.