Protocycloceratidae

Protcycloceratidae is an extinct family of slender, commonly annulate, members of the

• Lower Ordovician
  , cosmopolitan.
• Catoraphiceras. Annulate orthoconic shells with a circular to slightly depressed cross section, marginal siphuncle, and sutures characterized by a deep ventral lobe. Upper Lower Ordocician, widespread.
• flanges
  ; siphuncle submarginal, relatively narrow, segments expanded, necks very short, connecting rings thin. Lower/Upper Ordocian, Australia.
• Ectocycloceras. Compressed exogastric annulated cyrtocones with straight transevse sutures and a small marginal siphuncle composed of concave segments. Lower/Middle Ordovician, Eastern North America.
• Endocycloceras. Rapidly expanding endogastric forms with well-developed surface annulations and a marginal siphuncle composed of concave segments. Sutures straight and transverse. Early Lower Ordovician (Gasconadian Stage) of North America.
• Kyminoceras. Generally small, straight annulated shells with straight transverse sutures, circular cross section, and a small ventral siphuncle. However connecting rings in the type appear to be thin, putting its inclusion in question.
• Notocycloceras. Straight shells with transverse annuli, apparently straight transverse sutures, and a comparatively large ventro-marginal siphuncle composed of faintly concave segments. Upper Lower Ordovician, Australia.
• Rudolfoceras. A genus of moderately expanding, annulated slender exogastruc shells with generally straight sutures and tubular siphuncle that may be in contact with the ventral margin. May include species in which sutures are faintly sinuous. Upper Lower Ordovician, Champlain Valley, North America.
• Apocrinoceras, in which siphuncle segments are expanded and septal necks weakly cyrtochoanitic, included in the Protocycloceratidae in the Treatise (Furnish & Glenister 1964), is included in the Apocrinoceratidae (Flower, 1964).