Balanus concavus   scale in cms CLASS - Cirripedia ORDER - Thoracica FAMILY - Balanidae GENUS/SPECIES - Balanus concavus ROCK UNIT - none AGE - Recent LOCALITY - LaJolla, California, USA Barnacles are unusual arthropods. Most arthropods are mobile during life, but barnacles cement themselves to hard surfaces (rocks, shell surfaces, boat hulls) and stay wherever that surface is. Barnacles do NOT molt as most arthropods. Instead, they live in the same set of pyramid shaped plates all their adult life. Barnacles are a common fossil component of many Cenozoic deposits (last 65 million years), because of the heavily calcified plates. PRI 2824E