Balanus concavus   scale in cms CLASS - Cirripedia ORDER - Thoracica FAMILY - Balanidae GENUS/SPECIES - Balanus concavus (on Chesapecten madisonius ) ROCK UNIT - Yorktown Formation AGE - Pliocene (about 4 million years old) LOCALITY - Hotel Marl Pit, Yorktown, Virginia, USA Large scallop shells provided an excellent surface for barnacles to live on. Many barnacles from the Yorktown Formation. reach enormous sizes (over 6 cm high). When barnacles encrust living scallops, they affect the growth of the shell (barnacle scars). PRI 2824E