The Sphenodiscus lenticularis ammonite found in the Fox Hills Formation of South Dakota, U.S.A. Originally named by Owen in 1852 the genus Ammonites. This creature swam the seas of the Upper Cretaceous. The suture pattern shown in the images was once covered by an iridescent shell. See this link the University of Wisconsin for an example: http://pages.cs.wisc.edu/~ifischer/Collections/Fossils/Images/sphenodiscus2.jpg
Thanks to Mark Palatas for letting me photograph this specimen in his collection.
Learn more at Wikipedia: http://en.wikipedia.org/wiki/Sphenodiscus
