The rare species "P. cobbi", described by James Hall in 1859 based on fossils recovered in New York, USA, was the first species of "Pterygotus" to be described from outside of Scotland and expanded the known range of the genus considerably. Hall described this new species alongside two other North American species; "P. macrophthalmus" (now referred to "Acutiramus") and "P. osborni" (later synonymized with "P. macrophthalmus"). 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").