THEIB EXTENSIVE GEOGRAPHICAL DISTRIBUTION. 457 



PJiacops caudatus, and Calymene Blimenbachii, together 

 with corals of the genera Catenipora, Porites, and Cysti- 

 pJiyllum ; the organic remains partake rather of the character 

 of the lower Silurian formations beneath, than of the Upper 

 Ludlow above. 



The Wenlock Shale contains many fossils; among the 

 corals, Cystipliyllum and Porites prevail; the brachiopods 

 are represented by the genera Lingula and Atrypa ; many 

 of the other Silurian species extend into this shale, which 

 attains a thickness of 700 feet. 



II. THE LOWER SILURIAN ROCKS. 



These have been subdivided into two portions, the Caradoc 

 sandstone and the Llandeilo flags : the former is best deve- 

 loped in the vicinity of the Caradoc hills, in Shropshire; the 

 latter in Caermarthenshire. 



THE Oaradoc sandstone. The characteristic fossils of 

 this rock consist of various species of Atrypa, Leptcena, 

 Orthis, Pentamerus, Bellerophon, and Orthoceras, with Ten- 

 faculties, supposed to be the sheath of a worm nearly allied 

 to Serpula. In the Caradoc of Gloucestershire many beautiful 

 corals are found. 



THE Llandeilo flags. These contain, as their charac- 

 teristic shells, several species of Euompnalus, Lituites, 

 &c., gigantic trilobites, AsapTius Buchii, and Asaphus tyran- 

 nus, with numerous smaller forms belonging to the genus 

 Trinucleus. In the fine shales of this formation Graptolites 

 are very abundant. 



DISTRIBUTION OF THE SILURIAN GROUP. The Silurian 

 strata have been discovered in various parts of the Continent. 

 Sir R. Murchison found them in Belgium, the banks of the 

 Rhine, Westphalia, and Nassau ; in the north of Germany, 

 the extreme parts of Russia, and at the very confines of the 

 old world; the American geologists have shown that they 

 are extensively developed in the lake region of North 

 America.* 



* In his great work on the Silurian Sy8tem, Sir R. Murchison has given 

 a profusion of sections, numerous beautiful plates of fossUs, and a largs 

 geological map of the Silurian region of England and Wales 



