I was in Santa Monica the other day, so I looked for a fossil shop hallucigenia told me about: http://jurassicgallery.com/ They had quite an impressive array of fossils of various sorts, including a lot of shelled cephs. Being in Santa Monica, their prices were a bit steeper than the rock seller at the Hilo farmer's market where I got my last fossils, but they made up for it in selection... and they had a few "grab bag" baskets of ammonites and pieces of slate/shale (?) with ammonite impressions (and one thing that looked like a jellyfish; I was wondering if it's some Cambrian thingie...) Anyway, the guy who was there, Bobby, showed me one that was his "favorite ammonite" because it was cut right through the siphuncle, and is rather nice looking. Since I'm trying to learn a bit of the ID techniques, I was a bit mystified, since it looked a lot like a nautiloid to me. As it turns out, they're not terribly expert at IDing fossil cephs, since neither knew what a nautiloid is or how it differs from an ammonite, so I suspect I'm on the right track... anyway, I decided to buy it (probably unwise until I get a new job, but he suckered me in by discounting it because of my obvious enthusiasm...) They didn't have any record of where the fossil came from, but they did have a book Ammonites and Other Cephalopods if the Pierre Seaway Identification Guide (by Larson, Jorgensen, Farrar, and Larson) which described some nautiloids found there, and the closest one seemed to be Rutrephoceras dekayi var. montanaensis but I didn't have much to go on... it certainly seems to have a lot of nautiloid characteristics: central siphuncle, smooth septa, apparently simple sutures (although they're not really visible) and no visible ornamentation. I think it's involute, but it's hard to tell. Anyway, here's some pix, anyone want to take a guess, or tell me I've made a novice mistake?