Fig. 4 in The dorsal shell wall structure of Mesozoic ammonoids

Fig. 4. Construction of the prismatic reduced dorsal shell wall (A–E, G, median section, growth direction to the left, centrifugal; F, transversal section, cen- → trifugal). A. Phylloceras (Euphylloceras) sp., BSPG MAo-1769, early Albian, Cretaceous, Ambatolafia, Mahajanga Basin, NW Madagascar; A1, the dorsal shell wall consists of an outer wrinkle layer and a dorsal inner prismatic layer; A2, A3, organic wrinkles. B. Ptychophylloceras sp., BSPG MAn-4516, late Oxfordian, Jurassic, Sakaraha, Morondava Basin, SW Madagascar; B1, the dorsal shell wall consists of an outer wrinkle layer and a dorsal inner prismatic layer; B2, organic wrinkle. C–E, G. Desmoceras (Desmoceras) latidorsatum (Michelin, 1838), early Albian, Cretaceous, Ambatolafia, Mahajanga Basin, NW Madagascar. C. BSPG MAo-1783; C1, the dorsal shell wall forms a wrinkle layer-complex; C2, the wrinkle layer is enriched with organic material. D. BSPG MAo-1839, organic wrinkle. E. BSPG MAo-1788, the relief of an injury of the preceding whorl (i.e., forma aegra substructa of Hölder (1973) is overgrown by the outer wrinkle layer and compensated by the dorsal inner prismatic layer. G. BSPG MAo-1782, the wrinkle layer of the dorsal shell wall becomes prismatic. F. Neosilesites ambatolafrensis Collignon, 1963, BSPG MAo-1780, early Albian, Cretaceous, Ambatolafia, Mahajanga Basin, NW Madagascar; F1, at the umbilical seam, the outer prismatic layer and the nacreous layer of the attaching whorl wedge out; only the inner prismatic layer continues towards the spiral plane; the wrinkle layer wedges out towards the umbilical seam; F2, organic wrinkle. Abbreviations: dipl, dorsal inner prismatic layer; dipl 1/2, primary/secondary dorsal inner prismatic layer; dspl, dorsal septal prismatic layer; ipl, inner prismatic layer; ipl 1/2, primary/secondary inner prismatic layer; ncl, nacreous layer; ncl 1/2, primary/secondary nacreous layer; opl, outer prismatic layer; s, septum; spl, septal prismatic layer; wl, wrinkle layer.