Fig. 21 in The dorsal shell wall structure of Mesozoic ammonoids

Fig. 21. Construction of the seemingly complete dorsal shell wall (transversal section, centrifugal). Quenstedtoceras henrici Douville, 1912, BSPG MAn- 4768, early Callovian, Jurassic, Dubki near Saratov, Russia. A. At the contact of two whorls the outer prismatic layer and the nacreous layer of the attaching whorl wedge out but a "new" dorsal outer prismatic layer is formed towards the spiral plane which covers the wrinkle layer; the inner prismatic layer is continuous. B. The (dorsal) nacreous layer and the (dorsal) inner prismatic layer attach to the "new" dorsal outer prismatic layer which compensates the relief of the wrinkle layer. C. The dorsal shell wall consists of a wrinkle layer, a dorsal outer prismatic layer and a dorsal inner prismatic layer. D. The wrinkle layer left imprints in the dorsal outer prismatic layer. Abbreviations: dipl, dorsal inner prismatic layer; dopl, dorsal outer prismatic layer; ipl, inner prismatic layer, ncl, nacreous layer, opl, outer prismatic layer; wl, wrinkle layer.