NEW FINDINGS OF THE AMMONITE RHAEBOCERAS, INCLUDING A NEW SPECIES, FROM THE PIERRE SHALE OF EASTERN MONTANA

A third known specimen of the ammonite Rhaeboceras burkholderi Cobban, 1987, has been discovered in the upper part of the Pierre Shale of ea rly Maastrichtian age (Late Cretaceous) Baculites eliasi zone of Dawso n County, Montana. This finding extends both the geographical and stra tigraphic ranges of the species. In addition, R. cedarense new species is described from the Baculites baculus/grandis zone of the same regi on, which extends the stratigraphic range of the genus as currently re cognized. The new species is a small form of Rhaeboceras that appears to be closely related to and probably descended from R. burkholderi.