Fig. 3

Internal structure of the nepionic constriction and its primary varix (growth direction left). a–c Phyllopachyceras sp. (MAo-1771), a nepionic constriction and its primary varix, the post-nepionic shell (opl and ncl) is attached to its adoral end; b close-up of a, discontinuity of the nepionic constriction; c close-up of a, first appearance of the nacreous material which forms the primary varix. d–f Eotetragonites umbilicostriatus COLLIGNON (MAo-1773), d untypical nepionic constriction, this specimen forms an unusual adoral rib, the post-nepionic shell (opl and ncl) is attached to its adoral end; e close-up of d, discontinuity of the nepionic constriction; f close-up of d, first appearance of the nacreous material which forms the primary varix. aw ammonitella wall, dipl dorsal inner prismatic layer, ipl inner prismatic layer, ncl nacre layer, opl outer prismatic layer, pel pelicula, pvar primary varix, S septum, siph siphon, wl wrinkle layer