FAMILIES OF ROCKS 



Silurian times. We cannot in a brief summary of this 

 kind do more than indicate some of the more conspicuous 

 features. Corals began to spread through the clearer 

 seas : and reef building on a great scale took place, 

 generally some distance from the shores of the land. 

 Other life in great abundance and variety gathered upon 

 or about these reefs, and they became rich depositories of 

 the animals of their day. The Crinoids, which, though 

 animals, are sometimes called the lilies of the sea, 

 developed strongly ; sea urchins appeared, and forms akin 

 to barnacles. The ancestors of the pearl oyster and the 

 mussel date from Silurian times ; and so do the first 

 Ammonites, those creatures known to the youngest 

 collectors of fossils, and deriving their names from the 

 Canaanitish god Ammon, which had a ram's head. Sea 

 scorpions, sand fleas, king crabs, sea squirts, and worms 

 and fishes of various kinds haunted the Silurian seas. 

 The Silurian fish were most of them armed for defence, 

 some with plates of bone; some of them had their tails 

 stiffly joined to their backbones ; some had skin like a 

 prickly pear; some were not unlike the modern shark. 

 The plants have left us many records liverworts, ferns, 

 and club-like mosses. The growing vegetation gave a new 

 impulse to insect life plant-lice and cockchafers and the 

 scorpions we have named : and the vastness of their num- 

 bers is shown by the fact that they have outlasted the 

 changes and vicissitudes of a myriad generations. 



We may conclude this chapter by saying what we 

 imagine of the general appearance of our own islands to 

 have been. At the close of the Silurian period Britain 



210 



