Pelecypods, variously called as time went on, Cardium, then Protocardia, and then Nemocardium, have been described and figured from localities in the Gulf Coastal Plain since 1854. In the present article, for the first time, a specimen of Nemocardium nicolletti (Conrad) from Mississippi, found in the Matthews Landing marl member of the Porters Creek clay of Paleocene age, is identified and described. This M. nicolletti is compared with specimens from the Matthews Landing marl, Marengo County, Alabama, and at Naheola Landing, on the Tombigbee River, Choctaw County, Alabama. N. nicolletti ranges from supposed middle Eocene down into the Paleocene. No Tertiary species are now referred to the genus Protocardia. All Tertiary species, including N. nicolletti once referred to the genus Protocardia are now considered to belong under the genus Nemocardium Meek.