This is a 5 1/2" in diameter Sphenodiscus Ammonite.  It is from the Fox Hills Formation in South Dakota.   This spheno may not have much color, but would take a great polish.  It has 100% of the inner whirl and a solid suture covering.  It may have some rough edges on the spine, but it is still really nice.  Imagine the finished product!  Buy this and add a great addition to your collection.  Happy Bidding!