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.