ABSTRACT: Samples collected from the Guys Hill Formation (Yellow Limestone Group, middle Eocene) in the Seven Rivers region of northwestern Jamaica yielded a rich ostracode fauna, including eight new species. The samples were collected to determine the environment of deposition for the strata that contain both fully-terrestrial andmarine vertebrates, including seacows in the transition between land- and sea-faring morphologies. Adult and juvenile and male and female ostracode specimens were recognized, indicating an autochthonous thanatocoenosis. The dominance of cytherideid ostracodes, particularly the presence of significant numbers of noded cytherideids, indicates brackish conditions, at least during part of the chronostratigraphic interval, andmarine taxa, such as Orionina and Hemicythere, indicate fluctuating paleoenvironmental conditions,which is consistent with the range of vertebrate fossil faunas collected from the site. New species include Cytherelloidea nebuloreticulata, Paracytheridea jamaicaensis, P. inflata, Cythereis? iturraldei, Quadracythere robusta, Quadracythere polycosta, Hemicythere elongata, and Orionina anteroexposita.