This is fossilized Lobster from Gunn Point, Darwin, Northern Territory, Australia. It was formed after the area was catastrophically buried by an intense cyclone, with Mangrove trees in Calcium rich mud. This particular specimen, although not complete, is well preserved and measures 3 1/2″ x nearly 1 5/8″ x 1 1/2″.  It would make a great addition to any fossil collection.