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.