Bivalves, as well as Gastropods, show a big diversification of their families during the upper Cretaceous. In chalks of the English-Parisian pond, we do not find of rudistes, bivalves builders of cliffs, as in certain seas further south. The main Bivalves used for the stratigraphy of the chalk are Inocérames. This fossil group has a shell consisted of a thick coat of prisms of calcite. Most of the sorts possess streak with growth concentric. Certain sorts are huge, being able to reach one meter (the amateurs of fossils sometimes confusing them with imprints of dinosaurs). Even in the form of fragments, inocérames is the best stratigraphical fossils on the ground for small-dimension outcrops or in drilling in carrots.
Main things inocérames some chalk according to Tröger
Here is for example (slide F. Robaszynski) the sorts used for the stratigraphy of Cénomanien.
Inocérames-keys of Cénomanien in Campanien
Base of average Santonien:
F.O. of Cordiceramus cordiformis
Base of Santonien :
F.O. of Cladoceramus (= Platyceramus) undulatoplicatus, on the ground, St Valéry W, Representation Gallemi And al.On 2007, In and F Representation Walaszczyk and Cobban, on 2006.
Base of upper Coniacien:
F.O. of the kind Magadiceramus
Divided into two sub-zones:
Cremnoceramus crassus inconstans
Cremnoceramus waltersdorfensis hannovrensis
Cremnoceramus waltersdorfensis waltersdorfensi
Cremnoceramus deformis erectus
Inoceramus crippsi C)