THE SILURIAN PERIOD 



553 



distance from shore, and to have been of the barrier type. The 

 reef-forming habit appears to have been local rather than general, 

 for over large tracts corals are found scattered in a markedly 

 distributive fashion. 



The trilobites. No new families of trilobites appeared, though 

 some new genera were added and many species; but these did not 

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



Fig. 404. Silurian Trilobites: a, Sphcerexochus minis Bey., dorsal view; 

 6, Staurocephalus murchisoni Barr, dorsal view, showing the peculiar 

 globular anterior prolongation of the head; c, Deiphon forbesi Barr, 

 dorsal view of a peculiar trilobite having the pleural lobes much reduced; 

 d, Calymene niagarensis Hall, dorsal view of one of the commonest Si- 

 lurian trilobites; e, Cyphaspis christyi Hall, dorsal view. 



important, had already entered upon its decline numerically. The 

 highest forms were, however, structurally equal, and perhaps 

 superior, to any that preceded. 



Other marine invertebrates. 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. The peculiar Receptaculites family (Fig. 405). whose 

 affinities were long in doubt, was still present, though its climax 

 was passed. The graptolites had lost the importance they had in 

 Ordovician times, and by the end of the period neared extinction. 



