SILURIAN. 



85 



The following Table exhibits the main divisions of the 

 Silurian strata : — 



Wales and West of England. 



Lake District, Etc. 



Upper Ludlow. 



Kirkby Moor Flags. 



Aymestry Limestone. 



Lower Ludlow. 



Bannisdale Slates. 



Wenlock Limestone 



and Shale. 

 "Woolhope Beds. 



Denbighshire 

 Grits and Flags. 



Coniston Grits and Flags. 



Tarannon Shales. 



Upper Llandovery. 

 Lower Llandovery. 



Corwen Grits. 



Pale Shales or Slates. )Stockdale 

 GraptoliticMudstones) Shales. 

 Austwick Conglomerate 



(Basement-beds of Silurian). 



Upper Cambrian (Ordovician or Lower Silurian). 



The fossils of the Silurian System as here defined belong to 

 the "Third Fauna" of Barrande. They include the earliest 

 traces of Land Plants, also Sponges, and Graptolites, of which 

 the genus Mouograptits is conspicuous. Many Corals occur of 

 the genera Favosites, Heliolites, and Pctraia ; Crinoids (Encri- 

 nites) ; Phyllopods, including Discinocaris and Ceratiocaris ; 

 Trilobites, of the genera Acidaspis, Encrinurus, Phacops, 

 Hojjtaloiiotus, etc., as well as the Crustacea Pterygotus and 

 Eurypterns. The Brachiopods include Atrypa, LeptcBiia^ 

 Orthis, Pentameriis, Rhynchonella, and Strophomena ; the 

 Lamellibranchs include Cardiola and Ptennea ; among the 

 Gasteropods are Eiioniphaius, JMurcJiisonia, and Pleurotoniai'ia 

 (a form still existing) ; ^ and the Cephalopods include OrtJio- 

 ceras and Litiiites. The earliest traces of Fishes are found, 

 and these include Scaphaspis, Cephalaspis, Pteraspis, and 



1 See Dr. H. Woodward, G. Mag. 1885, p. 433. 



2 See the Thesaurus Si/uricus, by Dr. J. J. Bigsby, 1868 ; a work which con- 

 tains lists of the older fossils, up to the close of the Silurian period. 



