This prompted me to wonder if Spirula has an ink sack. Answer from TOLweb: "Radula absent. Ink sac reduced." The ink sack's consistent, the lack of radula seems wacky; I've never heard of a ceph with no radula before. But then again, I've never asked.
These authors seem to think the ink sac from the ammonoid mentioned below could be just remains of the "Black Layer" or other black deposit like those of modern nautilus. The black layer is made of the same "excretions of metabolic waste like cephalopod ink which all contain melanins"