Inoceramus gradually declined in abundance over a narrow stratigraphie interval of the Maastrichtian Stage and apparently went extinct synchronously across a wide range of depths and facies near the Globotruncana ganserri/Abathomphalus mayaroensis planktonic foram zonal boundary, well before the K/T boundary. This conclusion is based largely on the observed numeric density of prismatic shell fragments of Inoceramus through eight stratigraphie sections. If care is taken to minimize the possibility of contamination through physical and biological reworking, prism density seems to follow Inoceramus density and could be a powerful new biostratigraphic tool.