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.
The word usage examples above have been gathered from various sources to reflect current and historial usage. They do not represent the opinions of YourDictionary.com.