Hectoceras is a genus in the nautiloid cephalopod order Discosorida from the Upper Ordovician of Australia (Tasmania), known from a few isolated siphuncle specimens. The siphuncle specimens, which go with two described species, consist of gradually enlarging, expanded segments in a slender series, suggesting similarly slender shells, which themselves are unknown. The interiors of the siphuncles are filled with nested calcareous deposits that form endocones, conformable to the interior shape, leaving a narrow irregular central tube. Connecting with are apparently somewhat thick. Anomalously the bullettes, where the connecting rings attach to the previous septal openings are unswollen. The slight endogastric curvature, as interpreted by Teichert and Glenister, suggests inclusion in the Cyrtogomphoceratidae, or ancestral Reudemannoceratidae; however endosiphuncular deposits are otherwise lacking in both these families. The organic deposits within the siphuncle suggests it is more likely Hectoceras is a member of the Westonoceratidae to which it has been assigned. References Flower, R.H.and Curt Teichert 1957. The Cephalopod Order Discosorida. University of Kansas Paleontological Contributions, Mollusca, Article 6. July 1957 Teichert, C. 1964. Nautiloidea -Discosorida. Treatise on Invertebrate Paleontology Part K. Geological Soc of America and Univ. Kansas Press. Prehistoric cephalopod genera Ordovician animals Discosorida