Although sometimes treated as synonyms, Eurypterus remipes (Fiddlers Green Formation) and Eurypterus lacustris (Williamsville Formation) have usually been considered separate species. Conclusions have often been based on statistical methods used to examine sets of carapace measurements. Here, morphological evidence of the cuticular sculpture of the telson margin and the prominent embayment on the lateral carapace margin supports their status as separate species. These differences are consistent throughout ontogeny and between sexual dimorphs. In the interest of establishing taxonomic stability and of providing standards for comparison of morphology, neotypes have been selected for both E. remipes and E. lacustris. Two new morphological characters from the type A genital appendage and the operculum support a close relationship between E. remipes and E. lacustris. New and earlier evidence suggest that the population representing E. remipes developed into the slightly younger E. lacustris population through the process of heterochrony, and that they represent chronospecies rather than evolutionary species.