Ichthyosourus or Ichthyosaurus: How To spell?

The word Ichthyosourus is misspelled against Ichthyosaurus, a noun meaning "An extinct genus of marine reptiles; -- so named from their short, biconcave vertebrae, resembling those of fishes. Several species, varying in length from ten to thirty feet, are known from the Liassic, Oolitic, and Cretaceous formations."