Describes the late Middle Devonian ostracodes found in two selected diamond drill cores from Lambton County, Ontario. The study area has lithologies and ostracode fauna typical of other similarly aged strata of the Michigan Basin, and the study permits stratigraphic correlation across an area extending over south-western Ontario, western New York, northern Ohio, and the southern peninsula of Michigan. The introductory sections discuss ostracode faunas of the Michigan Basin & bordering areas and the ostracode biostratigraphy of the Hamilton Group strata. The main section presents a systematic palaeontology of ostracode species, with species descriptions arranged by order, family, and genus.