class CALYPTOPTOMATIDIDA (Cambrian to Permian)
The hyolithids; simple conical or pyramidal shells, with the aperture usually closed by an operculum. Near the base of some Lower Cambrian transgressive sequences, hyolithids occur in shallow marine beds under those containing the first local occurrence of trilobites; and for this reason (only) some authors consider that the hyolithids may extend into the Precambrian.
class CRICOCONARIDA (Ordovician to Carboniferous)
The tentaculitids and cornulitids; conical shells with exterior ornament of transverse rings.
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