I wrote a response 4 days ago to "
" which goes a long way to providing an answer to your question (it is a septarian nodule). Suggest you read it first. Big septarian nodules contract internally more than small ones so the network of secondary calcite-filled veins inside tends to be more prominent the larger the concretion. Your nodule, being quite small, has only small shrinkage fissures and it is along these that it has broken - observe the thin crystalline infill and slight yellowish discolouration. You might have had a better result trying to split it parallel to the original bedding (which would have been along its broadest axis) as shells and other objects destined to become fossils tend to lie flat on the sea floor. Pity about that ammonite. You probably have nothing to lose by cracking open the two pieces you have to find out if there are any more bits and pieces inside. "Fossils" can be hard or soft or anything in between (even a hollow cast counts). It depends upon what has happened to it after burial. It is sometimes a difficult task to remove well-cemented matrix from a delicate shell or bone. When you can, go back to the locality and I am sure you will discover lots more, now you have your eye trained.