Cassinoceras ("Cassin Horn") is a genus of nautiloids belonging to the endocerid family Piloceratidae that comes from the late Early Ordovician of eastern North America and adjacent territories. Cassinoceras is characterized by short, laterally compressed, rapidly expanding shells in which the upper, dorsal, surface has a convex curvature and diverges strongly from the essentially straight lower, ventral surface. The siphuncle which entirely fills the apical portion at the back of the shell is also compressed and vertically expanding, and is filled with simple endocones with a complex system of endosiphuncular blades through which a flattened endosiphuncular tube with widely spaced partitions runs. The shell of Cassinoceras grew to about 25 to 30 cm long.