LIFE 



399 



u Ti-asc in abundance, corals acquired the habit of associating them- 

 selves together. This resulted in the formation of reefs. The 

 known n-efs appear to have been of the barrier type formed some 

 distance from shore. The reef-forming habit appears to have been 

 local rather than general. 



Among trilobites, no new families appeared, though there were 

 some new genera and many new species; but the new forms did not 







Fig. 35.5. SILURIAN TRILOBITES: a, Sphcerexochns mirns Bey., dorsal view; b, 

 Stawocepkahu nmrcliisoni Barr, dorsal view, showing the peculiar globular anterior 

 prolongation of the head; c, Deiphonforbesl Barr, dorsal view of a peculiar trilobite 

 having the pleural lobes much reduced; d, Calymene niagarensis Hall, dorsal view 

 I one of the commonest Silurian trilobites; e, Cyphaspis chrislyi Hall, dorsal view. 



offset the disappearance of old ones, and the class, though still 

 important, had already begun its decline. The highest forms were, 

 however, equal, if not superior, to any that preceded. 



Sponges flourished. There was a prolific field of them in western 

 Tennessee, where the conditions were not only congenial to their 

 growth, but favorable for their preservation. Graptolites had lost 

 the importance they had in Ordovician times, and by the end of the 

 period neared extinction. Sea-worms are recorded through their 

 jaws, tracks, and burrows, and by the calcareous tubes which some 

 of them secreted. 



In the earlier and mid-Silurian deposits few relics of fishes have 

 been found, and these few are very imperfect; but in the upper part 

 of the system their remains are not rare. 



