Fig. 14 in The dorsal shell wall structure of Mesozoic ammonoids

Fig. 14. Construction of the nacreous reduced dorsal shell wall (A–C, E, median section, growth direction to the right, centrifugal; D, transversal section, centrifugal). A. Aspidoceras sp., BSPG MAn-3193, late Oxfordian, Jurassic, Sakaraha, Morondava Basin, SW Madagascar; the dorsal shell wall consists of a primary dorsal inner prismatic layer, a secondary dorsal nacreous layer and a secondary dorsal inner prismatic layer; the dorsal shell wall becomes thinner towards the aperture (A1–A3) and vanishes completely (A4). B. Eupachydiscus sp., BSPG MAo-1831, Campanian, Cretaceous, Teshio-Nakagawa → area, Hokkaido, Japan; the early secondary nacreous layer is part of the secondary inner prismatic layer. C. Euaspidoceras sp. 2, BSPG MAn-4751, late Oxfordian, Jurassic, Sakaraha, Morondava Basin, SW Madagascar; same as in B. D. Colombiceras sp., BSPG MAo-1884, Aptian, Cretaceous, Caucasus region, Russia; the dorsal shell wall cover of the flanks of the preceding whorl consists of a dorsal outer prismatic layer, a primary dorsal inner prismatic layer, a secondary dorsal nacreous layer (and a secondary dorsal inner prismatic layer). E. Desmoceras (Desmoceras) latidorsatum (Michelin, 1838), BSPG MAo-1787, early Albian, Cretaceous, Ambatolafia, Mahajanga Basin, NW Madagascar; the dorsal shell wall can develop nacreous material within the dorsal inner prismatic layer. Abbreviations: dipl 1/2, primary/secondary dorsal inner prismatic layer; dncl 2, secondary dorsal nacreous layer; hbl, heringbone layer; if, infilling; ipl, inner prismatic layer; ncl, nacreous layer; opl, outer prismatic layer; wl, wrinkle layer.