Heteromorphs were loosely coiled to uncoiled ammonites. Ammonites were cephalopods (like squids and octopi), but with an externally shell. They were abundant marine organisms during the Jurassic and Cretaceous Periods.
Ammonite, approx. 3" in dia., Cretaceous, Madagascar. There are two specimens available; price is for each. In the photos, the specimens shown are remain on either the left (with a small yellow circular sticker on the cut end) or the right side of the screen. Select via the pull-down list to order.
Learn more about ammonites from the Digital Atlas of Ancient Life: https://www.digitalatlasofancientlife.org/learn/mollusca/cephalopoda/ammonoidea/.