LIFE SYSTEM 389 



Africa, South America, and Australia. In all of these areas, as also 

 in North America, the fossils resemble those of the northern Euro- 

 pean region, rather than those of the southern. In general the 

 Silurian rocks are less extensively exposed at the surface than the 

 Ordovician. 



Close of the Silurian. — In North America the Silurian passed so 

 gradually and gently into the Devonian, that it is difficult to draw 

 the line between the two systems. Some disturbances, however, 

 took place in Ireland, Wales, and the north of England, for in these 

 localities the Devonian lies unconformably upon the Silurian, In 

 other parts of Europe the transition was gradual. 



The Life of the Silurian 



Silurian life is the continuation and advance of the same organic 

 system as nourished in the Ordovician, certain groups diminishing, 

 others expanding; and some new groups now make their first 

 appearance. 



Plants. — Our knowledge concerning the land vegetation of the 

 Silurian is not much more definite than concerning that of the 

 Ordovician. Most of the remains referred to land plants are of 

 disputable character ; the best authenticated is a fern {Nenropteris) 

 from the Silurian of France. 



Spongida are still common. A wide-spread form is Astylospongia 

 (PI. Ill, Fig. 1). 



Coelenterata. — The Graptolites have greatly diminished, espe- 

 cially the branching forms and those with two or more rows of 

 cells. Those that persist are, for the most part, straight and 

 simple (III, 2). The Hydroid Corals, on the other hand, such 

 as Heliolites, become important elements of marine life and in the 

 formation of the reefs. The true Corah likewise increase largely, 

 and play a more important role than in the preceding period. 

 The increase is principally in the enlarged number of species be- 

 longing to much the same genera. Favosites is a characteristic 

 new genus (see III, 3), and Haly sites, the chain coral, is much 

 commoner than before. 



