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I have some research to do I guess it's time to buy a book!
I have a few 'nodules' which are vague ammonite shaped, with a central short 'stalk' and an cracked eggshell like surface. Do you know if the eggshell-like surface is part of the preserved ammonite or just the nodule crust ?
It is likely just the nodule crust. If you can see impressions of ribs or some flakes of shell material it could be coming off the fossil inside. We get alot of septarian nodules around here, they are nodules with calcite crystals growing in a star pattern radiating from the center of the nodule. They form a cracked egg looking surface on the outside of the nodule, all the museums around here have them brought in by people thinking they are dinosaur eggs! usually any fossil inside is destroyed by the growth of the calcite crystals.
This is a Jurassic (Bajocian) Stemmatoceras (looks alot like some of those you found) the inner whorls have been destoyed by the calcite growth. The flat area in the center is the surface of a vein of calcite.