456 SPECIAL GEOLOGY. 



LOWER SILURIAN ROCKS. 



3. ") ("Flags of shelly limestone ( 



Caradoc ^Caradoc sandstones.^ and sandstone with white cu uullc> 



formation.) ( freestone. ( Thickness 2500 feet 



Llandeilo JLlandeilo flags. { Dark coloured calcareous! M ^ a ^^ 

 formation.) < fla S"- ( 1200 feet. 



I. UPPEE SILTJEIAN EOCKS. 



The Upper Ludlow. The first member of the upper series 

 consists of a grey calcareous sandstone containing the spines, 

 scales, and teeth of fishes of the genera Sphagodus, Ptery- 

 gotus, Plectrodus, Sclerodus, Theloclus, and Onchus. The 

 shells belong to the genera Avicula, Cypricardia, Leptcena, 

 Atrypa, Orthis orbicularis (1), and Terebratula navicula (2), 

 (fig. 300), are very abundant in some beds. 



The Aymestry limestone consists of a sub crystalline 

 argillaceous limestone, about fifty feet thick, in which Pen- 

 iamerus Knightii (4) is very abundant ; it also occurs in the 

 lower Ludlow: this singular brachiopod was found in 

 myriads, dispersed through a white limestone of the upper 

 Silurian group on the banks of the Is, on the eastern flank 

 of the Urals, in Russia. The Aymestry limestone contains, 

 likewise, Lingula Lewesii, Terebratula Wilsoni, and Atrypa 

 affinis (5) ; the latter shell has a wide vertical range, being 

 common to all the Silurian stages, the Llandeilo flags 

 excepted. 



The Lower Ludlow Shale consists of a dark grey argil- 

 laceous deposit, containing Graptolithus Ludensis, a form of 

 Silurian zoophyte, supposed to be allied to the Pennatula, or 

 sea-pen of our seas; here, likewise, are found Orthoceras 

 Ludensis, Lituites giganteus (7), and Phragmoceras ventrico- 

 sum (6), (fig. 300.) Homalonotus delphinocephalus (fig. 176) 

 is the trilobate common to this stage and the Wenlock lime- 

 stone, and is remarkable for having the longitudinal furrows 

 of the -carapace nearly obliterated, and forming a type of 

 this singular family of Crustacea, which is special to the 

 Silurian rocks. 



The Wenlock Formation. The lower division, comprising 

 the Wenlock formation, is subdivided into two portions, the 

 upper being the Wenlock, or Dudley limestone, and the 

 lower the Wenlock shale. Its characteristic fossils are 



