Two further species that remain assigned to the genus to this day would be described from England during the 19th century; "P. ludensis" of Pridoli (Late Silurian) age and "P. arcuatus" of Ludlow (Late Silurian) age, along with a North American species, "P. cobbi", from the Pridoli of the United States and Canada. "P. arcuatus" was originally described from a syntypic series (a series of specimens out of which a particular holotype is not designated) of fossil remains by John William Salter, containing five separate fossil specimens. In 1961, 102 years after its description, Erik N. Kjellesvig-Waering noted that only one of these specimens (Number 89587 of the Geological Survey and Museum of London, a free ramus of a chelicera) might truly be referrable to this distinct species as the other fossils (tergites, coxae and indeterminable fragments) might actually represent fossils of other species due to not being diagnostic enough. "P. ludensis", described by Salter in 1859, can be distinguished from other species by the more developed and prolonged keel (or ridge) running along the center of the telson from its beginning to the tail spike.