 The ascocerid shell, or conch, is the thinnest and most fragile of any orthocone or cyrtocone of comparable size, which accounts for their rarity.Often it is only the mature, ascoceroid portion that is found.The Ascocerida are derived from the michelinocerid family Clinocertidae in the early Middle Ordovician, possibly from "Clinoceras" through such slender forms as "Montyoceras" and "Hebetoceras".