Fig. 5 in The dorsal shell wall structure of Mesozoic ammonoids

Fig. 5. Construction of the prismatic reduced dorsal shell wall (median section, growth direction to the left, centrifugal). A. Desmoceras (Desmoceras) latidorsatum (Michelin, 1838), BSPG MAo-1786, early Albian, Cretaceous, Ambatolafia, Mahajanga Basin, NW Madagascar; A1, the early dorsal shell wall consists of a smooth organic layer and (prismatic) septal mural parts; A2, later in ontogeny, the septal mural parts extend and seem to form the first dorsal inner prismatic layer; A3, the prismatic mural part of a nacreous septum can form an own layer which is separated from the ventral inner prismatic layer; the nacreous and prismatic materials of a septum merge. B. Neosilesites ambatolafiensis Collignon,1963, BSPG MAo-1779, early Albian, Cretaceous, Ambatolafia, Mahajanga Basin, NW Madagascar; the prismatic mural part of a nacreous septum can form an own layer which is separated from the dorsal inner prismatic layer; the nacreous and prismatic materials of a septum merge. C. Argonauticeras besairiei Collignon, 1949, BSPG MAo- 1705, early Albian, Cretaceous, Ambatolafia, Mahajanga Basin, NW Madagascar; C1, the septal mural part seems to be the origin of the inner sub-layer of the dorsal inner prismatic layer; C2, close up of C1. Abbreviations: dipl, dorsal inner prismatic layer; dspl, dorsal septal prismatic layer; ipl, inner prismatic layer; ncl, nacreous layer; ol, organic layer; opl, outer prismatic layer; s, septum; spl, septal prismatic layer; wl, wrinkle layer.