ABSTRACT Inoceramids have been studied extensively over the last 200 years and, along with ammonites, have become an important tool for Late Cretaceous biostratigraphy. Geniculation, the abrupt change in convexity/inflation of the valve disc, occurs in many inoceramids and has been recognized for decades. But despite extensive inoceramid research, it remains a relatively under-documented phenomenon. To understand the phylogenetic relationships within this cosmopolitan bivalve family, it is necessary to recognize the range of intraspecies variation. “ Inoceramus ” nebrascensis is known to geniculate, but its morphological variability has not been effectively documented. This study identifies five morphotypes of “I.” nebrascensis from a single locality of the Late Campanian Pierre Shale in South Dakota. The most common form is non-geniculated (Type I). The four geniculated forms include: marked positive geniculation (Type II); slight positive geniculation followed promptly by marked negative geniculation (Type III); marked positive geniculation generating a conspicuous neck, followed by marked negative geniculation (Type IV); extreme positive geniculation with a “butterflied” configuration of the pre-adult shell (Type V). This study also argues for a three-stage shell development for this species (juvenile, intermediate, and adult) rather than the two stages described in previous studies.