Morphological studies can be used to examine paleoecological issues, such as the effects of environmental change on organisms. Much has been written on extinction caused by environmental change; however, relatively little research has been directed at species which survive these changes. The Marvacrassatella (Mollusca: Bivalvia) lineage is widely distributed and abundant in space and time. This study concentrates on the latter portion of the lineage. Five previously recognized groups of specimens, which have been thought to possibly represent three to five species, survived three episodes of habitat alteration over three million years. Approximately 800 specimens representing seven formations (Yorktown, Duplin, Raysor, Jackson Bluff, Tamiami, Chowan River, and James City) and three members (Rushmere and Moore House Member of the Yorktown Formation, and the Edenhouse Member of the Chowan River Formation) from the early late Pliocene to the early Pleistocene from Virginia to Florida were examined. Eighteen quantitative measurements were made on each valve. Principal-components analysis assessed shape variation within the Marvacrassatella lineage. Canonical discriminant analysis on geometric means produced two canonical variables that explain the separation among species. Most changes in shell and muscle shape occurred in adductor muscle dimensions, pedal retractor height, and the distance from the pallial line to the ventral margin, which have been overlooked in previous studies. Although the morphospace for each species overlaps, each has its own unique morphology. Analyses reveal morphological stability in the early late Pliocene, followed by an extinction event and a possible migration event in the late Pliocene, with later events leading to the complete extinction of the Marvacrassatella lineage in the early Pleistocene. / Master of Science