In ammonoids, a soft body organ (possibly a supracephalic mantle fold), extending from
 the conch aperture secreted aragonitic wrinkles, forming a layer on the surface of the pre-
 ceding whorl.  The dorsal shell wall consists of the outer and inner components which
 were deposited sequentially, beginning at the aperture of the living chamber inwards.  The
 dorsal wall attains its full thickness near the last septum.  The outer component is visible
 in the apertural region and is smooth or wrinkled; it is called the wrinkled layer in the lat-
 ter case. The wrinkles may be continuous, interrupted, or form isolated patches arranged
 in rows.  The wrinkles are usually triangular in cross section.  A further stage of dorsal
 wall development involves filling in the space between the apices of triangles, und then
 adding one or more inner prismatic layers from the inside of the living chamber.  This pat-
 tern occurs at least in the postembryonic stage of all genera studied, belonging to five
 suborders of Ammonoidea ranging from Late Carboniferous to Late Cretaceous.  In many
 genera, the outer component of the dorsal shell wall exhibits remarkable ontogenetic
 change in its ultrastructure and micro-ornament. lt may be compared with the black film
 of Recent Nautilus shells with respect to place of formation.  The outer component of the
 amrnonoid dorsal shell wall is regarded as a product of organic secretion and carbonate
 precipitation in the area of the supracephalic mantle fold.