Pterygotus monroensis from the Pittsford and Harris Hill members of the Silurian Vernon Formation, New York State, USA, is here assigned to Erettopterus osiliensis, previously known only from Saaremaa, Estonia, based on new material (carapaces, chelicerae, a prosomal appendage VI, several metastoma, genital opercula with appendages, an almost complete opisthosoma, and several bilobed telsons), and re-examination of the original specimens. Telson morphology indicates assignment to Erettopterus or Truncatiramus, but the differences between these two genera are here interpreted as insignificant, and all species are treated as belonging to Erettopterus. The much rarer Pterygotus? sarlei n. sp., a possibly basal pterygotid, also from the Pittsford Member, is based on two telsons with pterygotid-like marginal cuticular sculpture. The telson shape is unlike that of any other adult pterygotid; it is relatively similar to juvenile pterygotids and Slimonia, but lacks the long posterior spine found in Slimonia. These systematic revisions have implications for other pterygotid taxa. Pterygotus impacatus, from Estonia, is a junior synonym of E. osiliensis. P. waylandsmithi, from slightly higher in the Vernon Formation, and is transferred to Erettopterus based on similarities of the chelicerae to E. osiliensis.

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