My best bet on this specimen is a cephalopod, perhaps a Mooreoceras. It tapers ever so slightly, whereas a sea pen would be a much sharper. However, there are no discernible septal markings on the shell. The shape is nearly a circle in cross section. It measures 18mm in diameter and 50mm in total length. There is a bit of shell material left on at least 1/4 of the specimen.