Zittelloceras
Zittelloceras is an extinct genus of nauiloids from the order Oncocerida which are among a large group of once diverse and numerous shelled cephalopods, now represented by only a handful of species. "Zittleoceras", which is included in the family Oncoceratidae, is characterized by subcircular or sightly depressed cyrtoconic shells with ventral, suborthochoanitic empty siphuncles; the shell surface typically with crenulated transverse frills or distinct noncrenulated annulations. "Zittleoceras" has been found in middle and upper Ordovician sediments in North America and Scotland. (Sweet, 1964)