Fig. 22 in The dorsal shell wall structure of Mesozoic ammonoids

Fig. 22. Periostracal extension of the ventral shell wall (median section, growth direction to the right, centrifugal). A. Phylloceras (Euphylloceras) cf. velledae (Michelin, 1934), BSPG MAo-1770, early Albian, Cretaceous, Ambatolafia, Mahajanga Basin, NW Madagascar; a scythe-like extension of the ventral organic periostracum forms an organic radial lirae (Radtke and Keupp 2016); the dorsal inner prismatic layer of the succeeding whorl smoothes out the periostracum relief. B. Phylloceras (Euphylloceras) sp., BSPG MAo-1769, early Albian, Cretaceous, Ambatolafia, Mahajanga Basin, NW Madagascar; the same as in A. C. Phylloceras (Phylloceras) plicatum Neumayr, 1871, BSPG MAn-4509, late Oxfordian, Jurassic, Sakaraha, Morondava Basin, SW Madagascar; the ventral organic periostracum forms convex periostracal extensions; the dorsal inner prismatic layer of the succeeding whorl smoothes out the periostracum relief. D. Desmophyllites diphylloides (Forbes, 1846), BSPG MAo-1838, Campanian, Cretaceous, Teshio-Nakagawa area, Hokkaido, Japan; the same as in C. Abbreviations: dipl, dorsal inner prismatic layer; if, infilling; ipl, inner prismatic layer, ncl, nacreous layer, opl, outer prismatic layer, per, periostracum.