Several of you got this one partially correct. This is indeed a nice specimen of the late Cretaceous ammonite Placenticeras meeki. Ammonites were cephalopods that inhabited marine environments during much of the Mesozoic Era. They were exceedingly common in the Western Interior Seaway and can be found in abundance in the Pierre Shale today. Placenticeras is identified by its extremely complex sutures pictured in close up above.
However, this specimen is special. It belonged to long-time Rapid City resident Quentin Parsons. Quentin was an avid rock collector who found this specimen along the Cheyenne River. As you can see be the photo above, the specimen was encased in a concretion. Quentin spent considerable time extracting the ammonite from the concretion with hand tools. This is no small feat, even with pneumatic tools. The resulting product is an amazing example of Cretaceous marine life.
Special thanks to the Parsons family for donating this specimen in 2009.