Belongs within: Nautilosiphonata.
Protocycloceras is a genus of early cephalopods from the Lower Ordovician. Like many early cephalopods, Protocycloceras had a relatively small shell (perhaps about 5-10 cm long, guesstimating from the figures in Furnish & Glenister 1964) with numerous closely-spaced septa. Whether Protocycloceras was nektic like later cephalopods or still benthic is unknown, but the latter seems likely.
Characteristics (from Furnish & Glenister 1964): Strongly annulate orthocones with circular cross-section; annulations and sutures transverse and essentially straight; siphuncle approximately 0.3× conch diameter, ventral but not marginal; septal necks probably orthochoanitic, siphuncular segments tubular.
<==Protocycloceras Hyatt in Zittel 1900 [=Orotocycloceras (l. c.); incl. Dresseroceras Ulrich, Foerste et al. 1944]FG64 |--*P. lamarcki (Billings 1859) [=Orthoceras lamarcki]FG64 |--P. abnorme Ulrich, Foerste et al. 1944P68 |--P. affine Ulrich, Foerste et al. 1944P68 |--P. arkansasense Ulrich, Foerste et al. 1944P68 |--P. deprati Reed 1917K93 |--P. doniphonense Ulrich, Foerste et al. 1944P68 |--P. foerstei Butts 1926P68 |--P. geronticum Ulrich, Foerste et al. 1944P68 |--P. manitouense Ulrich, Foerste et al. 1944P68 |--P. odenvillense Ulrich, Foerste et al. 1944P68 |--P. philipsburgense Ulrich, Foerste et al. 1944P68 |--P. rushenseP68 | |--P. r. rushenseP68 | `--P. r. crebricinctum Girty 1911P68 |--P. smithvillense Ulrich, Foerste et al. 1944P68 |--P. whitfieldiFG64 `--P. wongi (Yü 1930)K93
*Type species of generic name indicated
References
[FG64] Furnish, W. M., & B. F. Glenister. 1964. Nautiloidea—Ellesmerocerida. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt K. Mollusca 3. Cephalopoda—General Features—Endoceratoidea—Actinoceratoidea—Nautiloidea—Bactritoidea pp. K129–K159. The Geological Society of America and the University of Kansas Press.
[K93] King, A. H. 1993. Mollusca: Cephalopoda (Nautiloidea). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 169–188. Chapman & Hall: London.
[P68] Purnell, L. R. 1968. Catalog of the type specimens of invertebrate fossils. Part I: Paleozoic Cephalopoda. United States National Museum Bulletin 262: 1–198.