Fig. 13 in The dorsal shell wall structure of Mesozoic ammonoids

Fig. 13. Construction of a secondary complete dorsal shell wall and the nacreous reduced dorsal shell wall (A, median section, growth direction to the left, cen- → trifugal; B, C, transversal section, centrifugal). A. Cleoniceras (Grycia) besairiei Collignon, 1949, BSPG PA-33582, early Albian, Cretaceous, Ambatolafia, Mahajanga Basin, NW Madagascar; in reaction to a forma aegra aptycha of Keupp (1977), the dorsal shell wall is secondarily complete; it consists of an outer wrinkle layer, a dorsal nacreous layer and a dorsal inner prismatic layer. B, C. Eupachydiscus sp., Campanian, Cretaceous, Teshio-Nakagawa area, Hokkaido, Japan. B. BSPG MAo-1832, the primary dorsal inner prismatic layer consists of two sub-layers. C. BSPG MAo-1834; C1, 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 primary and the secondary dorsal inner prismatic layer develop sub-layers; the primary dorsal inner prismatic layer shows a relief (i.e., "Ritzknoten"); C2, C3, umbilical-lateral, the primary inner prismatic layer forms cone-like elevations, i.e., "Ritzknoten"; C4, C5, the "Ritzknoten" reach up to the umbilical seam and the dorsum. Abbreviations: dipl, dorsal inner prismatic layer; dipl 1/2, primary/secondary dorsal inner prismatic layer; dncl, dorsal nacreous layer; dncl 2, secondary dorsal nacreous layer; if, infilling; ncl, nacreous layer; wl, wrinkle layer.