Orthoceras is a 400-million-year-old fossil. It is of the Cephalopoda genus of extinct nautiloid cephalopod endemic to Middle Ordovician-aged marine limestones of the Baltic States and Sweden. This genus is sometimes called Orthoceratites and is often misspelled as Orthocera, Orthocerus or Orthoceros. 

 

Scientific classification:

Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Order: Orthocerida
Family: Orthoceratidae
Genus: Orthoceras (Bruguière, 1789)
Species: O. regulare (Schlotheim 1820)

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Photos not to scale. All have been photographed with a ruler or a U.S. quarter for size reference.