The shell of
Palaeocycloceras (=?
Ctenoceras) is straight or faintly curved upwardly (exgastric) and somewhat compressed in cross section (slightly higher than wide). The surface has prominent sinuous annulations and growth lines. The
siphuncle is small, subcentral or between the center and venter. Necks are short and straight, connecting rings thin and slightly expanded at the anterior ends of the segments. In some the siphuncle has an apparently continuous parietal lining. No cameral deposits are known. ==References==