The loss of the power to coil, observed in the terminals of many declining series of gastropods from the Cambrian to the present time, and the similar loss of power among Natiloidea and Ammonoidea of many genetic series, as well as the ostraean form assumed by various declining series of pelecypods and by some brachiopods, may be cited as examples.
The groups of organisms utilized for zoning and correlation by different workers include brachiopods, pelecypods, cephalopods, corals, fishes and plants; and the results of the comparison of the faunas and floras of different areas where Carboniferous rocks occur are generalized in the table below.
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).
Each of the principal groups of true mollusca was represented: Pelecypods (Modioloides); Gasteropods (Scenella, Pleurotomaria, Trochonema); Pteropods (Hyolithellus, Hyo- tithes, Salterella); Cephalopods (Orthoceras, Cystoceras).
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.