Amongst the Cephalopods, the most striking feature is the rise and development of the Goniatites (Glyphioceras, Gastrioceras, &c.); straight-shelled forms still lived on in some variety (Orthoceras, Actinoceras), along with numerous nautiloids.
Each of the principal groups of true mollusca was represented: Pelecypods (Modioloides); Gasteropods (Scenella, Pleurotomaria, Trochonema); Pteropods (Hyolithellus, Hyo- tithes, Salterella); Cephalopods (Orthoceras, Cystoceras).
The upper divisions consist of bituminous limestones, clay-slates, alum-slate, and contain numerous species of trilobites of the genera Paradoxides, Conocoryphe, Agnostus, Sphaerophthalmus, Peltura, &c. The Ordovician formation occurs in two distinct facies - the one shaley and containing graptolites; the other calcareous, with brachiopods, trilobites, &c. The most constant of the calcareous divisions is the Orthoceras limestone, a red or grey limestone with Megalaspis and Orthoceras.
The subdivisions of the system may be grouped as follows: (I) Ceratopyge Limestone; (2) Lower Graptolite Shales and Orthoceras Limestone; (3) Middle Graptolite Shales, Chasmops and other Limestones, Trinucleus beds.
The lowest beds contain many fossils, including Phacops, Homalonotus, Leptocoelia, Spirifer, Chonetes, Orthothetes, Orthoceras, Bellerophon.