The distal tooth of the free ramus (the part of the claw that moves) was less prominent than in other species, which has been noted as similar to the distal tooth in the free ramus of "Acutiramus cummingsi". Although "P. cobbi" is based on poor fossil material, only known from a free ramus, it remains recognized as a distinct species on the account of being more similar to certain species discovered in the Czech Republic (such as "P. barrandei") than it is to other pterygotid species discovered in the "P. cobbi" locality (such as "Acutiramus macrophthalmus"). By 1859, 10 species had been assigned to the genus, and John William Salter recognized that it was possible to divide these species into subgenera based on the morphology of the telsons. Salter erected the subgenus "Pterygotus" ("Erettopterus") for species with a bilobed telson. Further subgenera would be named as more differences were noted between the species, such subgenera include "Pterygotus" ("Curviramus") and "Pterygotus" ("Acutiramus"), named in 1935 based upon features of the denticles (teeth) of the chelicerae. "Pterygotus" ("Curviramus") was later recognized as synonymous with "Pterygotus" ("Pterygotus") by Leif Størmer the same year, and "Erettopterus" and "Acutiramus" would be recognized as separate, but closely related, genera ("Erettopterus" by Erik N. Kjellesvig-Waering in 1961, and "Acutiramus" by Størmer in 1974). In 1912, the family Pterygotidae was erected by John Mason Clarke & Rudolf Ruedemann in 1912 to include the eurypterid genera "Pterygotus", "Slimonia", "Hughmilleria" and "Hastimima". The three latter genera would be reclassified as members of the Hughmilleriidae by Erik N. Kjellesvig-Waering in 1951, leaving "Pterygotus" and its former subgenera as the sole pterygotid eurypterids. Though early discoveries of "Pterygotus" were confined to England and North America, fossil finds throughout the 20th century would help establish that "Pterygotus" as a genus achieved a nearly cosmopolitan distribution.