The Antiarchi ("Opposite anus") were the 2nd most successful order of placoderms known, after the Arthrodira. The order's name was coined by Edward Drinker Cope, who, when examining some fossils that he thought were armored tunicates related to Chelysoma, mistakenly thought that the eye-hole was the mouth, and that the opening for the anal siphon was on the other side of the body, as opposed to having both oral and anal siphons together at one end.