COMMENCEMENT OF ORGANIC LIFE. 



35 



System, because largely developed at the surface of s dis- 

 trict of western England, formerly occupied by a people 

 whom the Roman historians called Silures. It is a se- 

 ries of sandstones, limestones, and beds of shale, (harden- 

 ed mud,) which are classed in the following sub-groups, 

 beginning with the undermost : — 1, Llandillo rocks (dark- 

 ish calcareous flagstones ;) 2 and 3, two groups called Ca- 

 radoc rocks ; 4, Wenlock shale ; 5, Wenlock limestone ; 

 6, Lower Ludlow rocks (shales and limestones ;) 7, Ay- 

 mestry limestone ; 8, Upper Ludlow rocks (shales and 

 limestone, chiefly micaceous.) From the lowest beds 

 upwards, there are polypiaria, though most prevalent in 

 the Wenlock limestone ; conchifera, a vast number of 

 genera, but all of the order brachipoda (including tere- 

 bratula, pentamerus, spirifer, or this, leptaena ;) mollusca, 

 of several orders and many genera (including turritella, 

 orthoceras, nautilus, bellerophon ;) Crustacea, all of them 

 trilobites (including trinucleus, asaphus, calamene.) A 

 little above the Llandillo rocks, there have been discover- 

 ed certain convo! i f ed forms, which are now established 

 as annelids, or sea-wo. "ns, a tribe of creatures still exist- 

 ing (nereidna and serpulina,) and which may often be 

 found beneath stones on a sea-beach. One of these, figured 

 by Mr. Murchison, is furnished with feet in vast numbers 

 all along its body, like a centipede. The occurrence of 

 annelids is important, on account of their character and 

 status in the animal kingdom. They are red-blooded and 

 hermaphrodite, and form a link of connexion between the 

 anmilosa (white-blooded worms) and a humble class of 

 the vertebrata.* The Wenlock limestone is most remarka- 

 ble amongst all the rocks of the Silurian system, for or- 

 ganic remains. Many slabs of it are wholly composed of 

 corals, shells, and trilobites, held together by shale. It 

 contains many genera of crinoidea and polypiaria, and it 

 is thought that some beds of it are wholly the production 

 of the latter creatures, or are, in other words, coral reefs 

 transformed by heat and pressure into rocks. Remains 

 of fishes, of a very minute size, have been detected by Mr. 

 Philips in the Aymestry limestone, being apparently the 

 first examples of vertebrated animals which breathed upon 

 our planet. In the upper Ludlow rocks, remains of six 

 genera of fish have been for a longer period known ; they 

 belong to the order of cartilaginous fishes, an order of 

 * Such as amphioxus and myxene. 



