Fig. 6
From: The internal structure of varices and constrictions in Jurassic and Cretaceous ammonoid shells
Parallel occurrence of varices and constrictions (growth direction left). The internal thickening is formed by the nacre layer; the inner prismatic layer becomes thinner in these areas. a–d Holcophylloceras sp. (MAn-4510), a in course of ontogeny, the varices are replaced by constrictions, b asymmetrical varix, c close-up of b, adoral end of varix, only the inner portions of the nacre layer seem to continue, d symmetric constriction, the outer prismatic layer forms the outer relief which is compensated by the dorsal inner prismatic layer. con constriction, dipl dorsal inner prismatic layer, ipl inner prismatic layer, ncl nacre layer, opl outer prismatic layer, S septum, var varix, wl wrinkle layer