Evolution within the Silurian cephalopod genusInversoceras

Specimens of the Silurian cephalopod genusInversoceras from the Wenlockian Slite Beds of Gotland, Sweden, are more curved and swollen than those from the Ludlowian Hemse Beds of Gotland. The former population is interpreted as a direct ancestor to the latter. The younger form,Inversoceras p. perversum, originally described by Barrande from Bohemia, includes both individuals with a fairly straight shell and with a slightly curved shell. The distinctly curved and swollen form from the Wenlock beds is herein described as a new subspecies,I. p. eoperversum. I. p. falciformis from the Ludlowian of Bohemia is excluded fromI. perversum and. treated as a distinct species. Its relationship to the olderI. p. eoperversum is discussed. This discussion also includesI. dayi from the Niagarian of North America.