In the seas, bony fish and crab-like decapods increased in numbers and variety, while pelecypods and gasteropods took the prominent place previously occupied by ammonites and belemnites, and, leaving behind such forms as Rudistes, Inoceramus, &c., they gradually developed in the direction of the modern regional groups.
Each of the principal groups of true mollusca was represented: Pelecypods (Modioloides); Gasteropods (Scenella, Pleurotomaria, Trochonema); Pteropods (Hyolithellus, Hyo- tithes, Salterella); Cephalopods (Orthoceras, Cystoceras).
Pelecypods among the true mollusca were increasing in numbers and importance (Aviculopecten, Posidonomya); Nucula, Carbonicola, Edmondia, Conocardium, Modiola, Gasteropods also were numerous (Murchisonia, Euomphalus, Naticopsis).
This idea originated in the discovery of a jelly-fish, gasteropods, and other organisms of a more or less marine type, and presenting some affinity with forms of Jurassic age.
Schrenck in the north of the Japanese Sea led to the discovery of no fewer than 256 species (Gasteropods, Brachiopods and Conchifers).