Didymoceras cheyennense

Didymoceras is an extinct genus of ammonite cephalopod from the Late Cretaceous epoch. It is one of the most bizarrely shaped genera, with a shell that spirals upwards into a loose, hooked tip. It is thought to have drifted in the water vertically, moving up and down. The genus name comes from Ancient Greek δίδυμος (dídumos), meaning "twin", and κέρας (céras), meaning "horn".