Perisphinctes is an extinct genus of ammonite cephalopod. They lived during the Jurassic Period http://pubs.usgs.gov/gip/geotime/fossils.html, and serve as an index fossil
Index fossils are fossils used to define and identify geologic periods . They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil...
for that time period.