Update! This specimen turned out to be a Schistoceras, my first found Ammonoid. Ammonoids are a type of Cephalopod with intricate septal chambers, having intricate patterns throughout. The outside of the creature was covered with the lines seen below. They also have the same basic shell shape as the Bellerophon gastropod, whereas Bellerophon has recently been debated as being true gastropods at all (Cherns et al., 2004). They could be a new type somewhere between Cephalopods and Gastropods. But this is still being debated in journals and no clear consensus has been reached.
While appearing to be a cephalopod, Bellerophon is a snail. It has existed since the Silurian and disappeared shortly after the great Permian Extinction. I’m not positive this is Bellerophon, but the growth lines visible resemble the genus.