I found this yesterday in Avoca, New York. It's about 3 cm long. The core looks like a cephalopod of some kind, but the exterior has me puzzled. If it were soft tissue, it wouldn't have preserved, but I have found impressions of a few others like this there and in another site over an hour away. The one field guide I have shows nothing like it and I may have to go hunt down the original 19th century sources. Any clues?