HANDBOOK OF FOSSILS. 91 



limestones, and conglomerates, varying greatly in different 

 districts, and containing few fossils. 



3. Devonian or Old Red Sandstone. To this age are assigned 

 a perplexing series of strata, the principal members of which 

 consist of (a) a thick limestone, well seen in the cliffs and 

 marble quarries of south Devon, and full of fossil-corals (e.g. 

 Favosites polymorpha [or cervicornis] ) Brachiopods, and Mol- 

 lusca, etc. 



b. A series of sandstones, slates, and limestones in North Devon 

 containing Trilobites (Phacops, Bronteus, etc. ), Brachiopods, and 

 other fossils. 



c. The Old Red Sandstone of Wales, the North of England, and 

 Scotland, consisting of red and grey sandstone and marly beds, 

 with remains of fish. 



These fish, unlike most now living, were more or less covered 

 with hard external plates, and possessed merely a cartilaginous 

 skeleton. In one set of individuals, indeed (Pterichthys), the 

 armour plates formed quite a little box. These creatures pro- 

 pelled themselves by means of two arm-like flippers, rather than 

 fins. They were but a few inches long, and appear pigmies in 

 contrast to the strange half-lobster-like crustucean, Pterygotus, 

 that lived with them, and attained sometimes as much as five 

 feet in length. 



4. Silurian. Named by Sir Roderick Murchison after a tribe of 

 Ancient Britons that dwelt in that part of Wales, where these rocks 

 were first observed. Some of Murchison's Lower Silurian beds 

 were included by Professor Sedgwick in his Cambrian, of which 

 we shall have to speak next ; and as these two geologists never 

 could agree on a divisional line between their respective forma- 

 tions, and since succeeding observers have followed sometimes 

 one and sometimes the other method of classification, consider- 

 able confusion has resulted. Here, however, for several reasons, 

 we propose to follow Sedgwick's arrangement ; and hence, under 

 the term Silurian, retain only Murchison's Upper beds. They 

 consist of a series of sandstones, gritstones, conglomerates, shales, 

 limestones, etc. 



Amongst the more important fossils, which are very abundant 

 in the limestones, are various corals (e.g. the Chain-coral 

 ffalysites), Star-fish, Crinoids, Trilobites (Phacops, etc.), Poly- 

 zoa, Brachiopods and Mollusca, especially Cephalopoda (Ortho- 

 ceras, Nautilus, etc.). 



These rocks occur principally in the border land between 

 England and Wales, and the adjacent counties ; but are also 



