Over the past 180 years, there has been occasional observation that museum collections of specimens identified as “Calymene senaria” or “Flexicalymene senaria” comprise more than one species. This was first recognized within a year after the species was named and has been remarked upon as recently as 2002. Perusal of compiled literature in concert with examination of new and historical collections at the American Museum of Natural History, the New York State Museum, Paleontological Research Institution, and Harvard Museum of Comparative Zoology, reveals that this history is due to the perpetuation of parallel conceptions of this species, in part because of the lack of an original type and relatively recent adoption of a neotype (Ross, 1967), as well as the application of the name to specimens from a wide range of localities. We review the taxonomic history of Flexicalymene senaria (Conrad, 1841) and provide a formal description of a second species, which we designate Flexicalymene trentonensis, n. sp. In addition we note that many specimens in museum collections of “senaria” also include specimens of the genus Gravicalymene. We correct taxonomic assignments for over 350 specimen lots housed in the above repositories, and describe how the clarification of this taxonomic history impacts our understanding of important faunal assemblages (Walcott-Rust Quarry, NY) and the reinterpretation of previously documented phyletic patterns.