Abstract: Placenticeras is widely distributed in the world during the late Cretaceous. Only are those forms such as Placenticeras meeki and Placenticeras intercalare, yielded in the Bearpaw Shale Formation from west Canada and in the Pierre Shale Formation from the United States，preserved as a gemstone, so-called Ammolite. Ammolite is characterized with excellent colors, it is also a kind of expensive gemstone, peculiar to the regions of the western Interior Sea of north America during the Cretaceous. It might be most likely related to the habits, taphonomic and diagenesis processes of Placenticeras meeki, which may be referred to soft bottom with high sedimentary rate, most of adult macroconch, well-preserved primitive micro-texture of aragonite, and its skeletal mineralogy solution and interaction peculiar to a shallow and periodic brackish-water environment.

Key words: North America, late Cretaceous, Ammonite, Ammolite