For more than 150 years, eurypterids from the Upper Silurian Bertie Formation of New York State and southern Ontario have been assigned to two species: Eurypterus remipes DeKay, 1825 and Eurypterus lacustris Harlan, 1834. These species have been distinguished on overall size and on the length–width ratios of the prosomas. To test the hypothesis that these taxa are simply ontogenetic variants of a single species, a biometrical study of more than 140 specimens from eight localities was performed. Bivariate plots of prosomal length versus width showed that E. remipes and E. lacustris morphs form a single ontogenetic growth series. Multivariate analysis using principal component analysis on nine measurement variables demonstrated that there is no basis for distinguishing between the two species on prosomal morphology. Accordingly E. lacustris is synonymized with E. remipes.