Bellerophon crassus

Bellerophon It's a genre of extinct marine mollusks, which lived from the Ordovician to the Lower Triassic, of uncertain position (Gastropoda or Monoplacophora). 

This genus is characterized by a shell:

• globose.
• convolute (each turn partially embraces the previous one).
• planispiral (symmetrically coiled). 
• It usually measures a couple of centimeters at most.
• its external surface is smooth, ornamented only by growth lines. It has a small ridge or ridge that runs along the midline of the shell.
• It superficially resembles that of a cephalopod but lacks septa.