THE SILURIAN PERIOD 



445 



were divided by horizontal partitions. Honeycomb corals were rare 

 in the Ordovician, but built coral reefs in the Silurian and Devonian. 

 (3) Cup corals (Enterolasma, Fig. 413 By C, E) were horn-shaped, 

 with a depression in the top. A peculiar cup coral of the period 

 (Goniophyllum, Fig. 413 E) was provided with a cover which con- 

 sisted of four triangular plates, hinged to the margins of the cup. The 

 covering was evidently for protection against enemies, but since the 

 genera which possessed it have no living representatives, it is probable 

 that the device was not successful. Cup corals occurred in the Or- 

 dovician and continued until the close of the Paleozoic; many of 

 these were separate individuals, while some were in hemispherical 

 colonies (Fig. 451 A, p. 480). (4) Organ-pipe corals (Syringopora, 

 Fig. 413 D) were similar to chain corals, but their cylindrical columns 

 were not attached along their entire length. 



Coral reefs date from the later Ordovician. Before this time simple 

 corals predominated, and even these were rare. When, however, com- 

 pound corals such as the honeycomb became abundant, the lime- 

 stone secreted by many generations, together with that of associated 

 animals such as brachiopods, gradually built up reefs at short distances 

 from the shores. Si- 

 lurian coral reefs were 

 seldom of great thick- 

 ness. 



Other Coelenter- 

 ates. — Stromatopora 

 were important reef 

 builders, but grapto- 

 lites no longer played 

 an important role in 

 America, and by the 

 end of the period 

 were practically ex- 

 tinct. Sponges (Fig. 

 414 ) are common in 

 certain beds, the peculiar family Receptaculites (Fig. 414 B) which 

 began in the Ordovician being not uncommon in some localities. 



Echinodermata 



Crinoids (Fig. 415 A, C) became so numerous in the Silurian that 

 their " joints " constitute an important part of the beds of certain 



A B 



Fig. 414. — Silurian sponges : A, Astrceospongia meniscus, 

 two views; B, Receptaculites oweni. 



