Fig. 12 in The dorsal shell wall structure of Mesozoic ammonoids

Fig. 12. Construction of the nacreous reduced dorsal shell wall (A, D3, transversal section, centrifugal, B, C, D1, D2, median section, growth direction → to the left, centrifugal). A. Perisphinctes (Kranaosphinctes) mahabokensis (Collignon, 1959), BSPG MAn-4835, late Oxfordian, Jurassic, Sakaraha, Morondava Basin, SW Madagascar; A1, the dorsal shell wall consists of a primary dorsal inner prismatic layer, a secondary dorsal nacreous layer and a secondary dorsal inner prismatic layer; A2, the secondary dorsal inner prismatic layer; A3, the primary dorsal inner prismatic layer. B. Kepplerites galilaeii Oppel, 1862), BSPG MAn-4783, early Callovian, Jurassic, Znamenka on Unzha River, Russia; same as in A1. C. Mirosphinctes sp. 1, BSPG MAn-1769, late Oxfordian, Jurassic, Sakaraha, Morondava Basin, SW Madagascar; the dorsal shell wall consists of a secondary dorsal nacreous layer and a secondary dorsal inner prismatic layer. D. Aspidoceras sp., BSPG MAn-4507, late Oxfordian, Jurassic, Sakaraha, Morondava Basin, SW Madagascar; D1, D2, the same as in A1; D3, at the umbilical seam multiple new shell layers are formed; the inner layers of the (dorsal) nacreous layer (dncl 1–3) and of the dorsal) inner prismatic layer (dipl 1–4) wedge out towards the spiral plane; the inner layers form the nacreous reduced dorsal shell wall. Abbreviations: dipl 1/2/3/4, primary/secondary/tertiary/quaternary dorsal inner prismatic layer; dncl 1/2/3/4, primary/secondary/tertiary/quaternary dorsal nacreous layer; dspl, dorsal septal prismatic layer; if, infilling; ipl, inner prismatic layer; ipl 1/2, primary/secondary inner prismatic layer; ncl, nacreous layer; ncl /2, primary/secondary nacreous layer; opl, outer prismatic layer; s, septum.