Transverse serial sections in a shell of Cyclothyris difformis (MNHN.F.A70934, Cap de la Hève, Normandy, France) with a length of 25.0 mm. Internal characters from the posterior shell to the level of the crura. Each section is labelled with a letter and the worn thickness between two successive sections is indicated in millimetre (this mention is valid for all sets of transverse serial sections). Line drawings: D. Gaspard.