342 THE BOOK OF THE ANIMAL KINGDOM 



anciently occupying territory in Shropshire, bordering on Wales. 

 The Ordovician rocks are noted for their curious fossil remains 

 known as Graptolites, which are creatures allied to the modern sea- 

 firs, discoverable on our coasts. Trilobites and some molluscs and 

 worms also thrived in Ordovician times. The thickness of these 

 rocks is calculated to be about 15,000 feet, and the system includes 

 grits, shales, slates, limestones and sandstones. The rocks are espe- 

 cially well developed in Shropshire and Eastern Wales. 



In the SILURIAN rocks, estimated at a thickness of some 7,000 feet, 

 we find, in addition to many of the fossils peculiar to the Cambrian 

 and Ordovician, remains of fishes and marine scorpions. These 

 rocks consist of sandstones, conglomerates, limestones, shales, mud- 

 stones and flagstones. Here are discovered traces of land-plants. 

 The fish appear towards the top of the series, and they are the earliest 

 vertebrate or backboned animals of which we have found fossil 

 remains. The name Silurian is associated with the tribe of the 

 Silures who, in Roman times, occupied ground in Shropshire and 

 Central and South Wales, where the rocks of this system are 

 typically developed. 



Next in order to the Silurian rocks come those of the DEVONIAN 

 system, attaining a thickness of about 5,000 feet, and found in 

 Devonshire, Cornwall, Wales and Scotland. The rocks called 

 Devonian are principally marine in origin, but the Old Red Sand- 

 stone was laid down at the same time, though under different con- 

 ditions. Remains of fish are so numerous in Devonian rocks that 

 the period during which they were deposited is commonly known as 

 "The age of fish." The fish of Devonian age are called ganoids on 

 account of the coat of armour with which nature had provided them 

 as a protection against their enemies. This armour was made up of 

 a series of bony plates which can be easily recognized in well- 

 preserved specimens. 



The CARBONIFEROUS system succeeds the Devonian, and its rocks 

 attain an approximate thickness of 12,000 feet. They are usually rich 

 in fossil remains, and they include the coal measures. The remains 

 of this system give evidence of great developments in life-forms 

 and an increasing variety thereof. Its limestones contain corals, 

 sea-lilies and shells of various species. In Carboniferous times there 

 were huge club-mosses and horsetail plants which must often have 

 reached a height of thirty or forty feet; tree-ferns were also numer- 

 ous. Some of the club-mosses and horsetails at present extant are 



