Thirty-five ostracode species were found at outcrops in western Prince Georges County, Md. Thirteen species are new, and the new genus Opimocythere of the Brachycytheridae is described. Only seven of the Brightseat species are known to occur in the overlying Aquia Formation. The presence of 13 Brightseat species in Gulf Coastal Plain deposits indicates correlation with the Kincaid and Clayton Formations, probably the upper parts of these units. The Brightseat is placed in the Globigerina daubjergensis-Globorotalia compressa zone of the Danian Stage, or lower Midwayan in the provincial terminology of Murray (1961). The Brightseat assemblages are dominated (average of 65 percent of individuals) by three species of Cytherideinae and two of Brachycytheridae. An inner sublittoral depositional depth is indicated.