The Protcycloceratidae is an extinct family of slender, commonly annulate, members of the cephalopod order Ellesmerocerida

Ellesmerocerida

The Ellesmerocerida is a order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.-Morphology:...

 that lived during the Early Ordovician

The Protocycloceratidae, named by Kobayashi (1935), are characterized by transversely ribbed, elongate shells that may be straight or slightly curved, as to be either endogastric or exogastric. (Flower 1964, Furnish & Glenister 1964). Since then the family has been revised in an emendation of the Ellesmerocerida (Kröger and Mutvie 2005) to include only those which have straight transverse sutures and siphuncles with concave segments and thick connecting rings

The Protocycloceratidae now includes the follow genera, with brief descriptions.

• Protocycloceras. Strongly annulate orthocones; cross section circular; annulations and sutures transverse and essentially straight; siphuncle tubular, ventral but not marginal, about 0.3 correspondinc shell diameter. L Ordovician, cosmopolitan.
• Ectocycloceras. Compressed exogastric annulated cyrtocones with straight transevse sutures and a small marginal siphuncle composed of concave segments. middle Lower Ordovician, Eastern North America.
• Endocycloceras. Rapidly expanding endogastric forms with well-developed surface annulations as a marginal siphuncle composed of concave segments. Sutures straight and transverse. Early Lower Ordovician (Gasconadian) North America.
• Kyminoceras. Generally small, straigtt annulated shells with straight transverse sutures, circular cross section, and small ventral siphuncle. However connecting rings in the type appear to be thin.
• 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 with faintly sinuous sutures. Upper Lower Ordovician (Cassinian
  Cassinian
  The Cassinian is the latest age of the Canadian Epoch when thought of temporally and the uppermost stage of the Canadian Series when thought of stratigraphically...
  
  ) Champlain Valley
  Champlain Valley
  The Champlain Valley is a region of the United States around Lake Champlain in Vermont and New York extending slightly into Quebec, Canada as part of the St. Lawrence River drainage basin drained northward by the Richelieu River into the St...
  
  , North America.

The following genera once included have been removed, key characters define them belonging elsewhare

• Apocrinoceras. Siphuncle segments are expanded,septal necks weakly cyrtochoanitic.
• Catoraphiceras. Ventral lobes are deeply rounded.
• Diastoloceras. Siphuncle segments aare expanded, connecting rings thin.

References

• Flower, R.H. 1964, The Nautiloid Order Ellesmeroceratida (Cephalopoda)

Memoir 12; New Mexico Bureau of Mines and Mineral Resources, Socorro, NM

• Furnish & Glenister 1964. Nautiloidea -Ellesmerocerida. Treatise on Invertebrate Paleontology Part K, Geological Society of America and University of Kansas Press.
• Kröger and Mutvie 2005; Nautiloids with multiple paired muscle scars from Early-Middle Ordovician of Baltoscandia. - Its bearing for taxonomic and phylogenetic reconstructions; Palaeontology, 48:1-11.