 The Choanoceratidae is established for the derived Upper Silurian "Choanoceras", characterized by septa that form deep symmetrical cones and a subcentral expanded siphuncle with cyrtochoanitic and recumbent septal necks at maturity.The middle and possible upper Ordovician ancestral forms are placed in the Hebetoceratidae, which includes the orthoconic "Hebetoceras" and cyrtoconic "Montyoceras".The fully mature breviconic ascocerid, with the juvenile longiconic portion discarded was no doubt a facile swimmer.