Spines and ribs
This is probably the most distinctive of the often highly-eccentric ammonites from the Lower Chalk. The image is two views of the same ammonite, showing how the ribs extend right across the keel (the edge of the ammonite). There are several similar forms of ammonite in the Lower Chalk and they can be distinguished by the spines, tubercles and ribs that may split (bifurcate).
Find out more about ammonites on the molluscs pages.