398 



SILURIAN PERIOD 



The prominence gained by corals (polyps) in suitable situations 

 is one of the notable features of the Silurian fauna. In the Ordovi- 

 cian period, simple forms predominated over compound, but the 

 ratio was now reversed. Among the notable types was the chain 



Fig. 352. SILURIAN CORALS AND BRYOZOANS: a-e are corals, a, Favosites 

 occidens Whit.; b, Syringopora verticillata Goldf.; c, Halysites catenulatus Linn.; d, 

 Goniophyllum pyramidale (His.) ; e, Zaphrentis umbonata Roming. Bryozoans, / 

 and g, Fenestella parmdipora Hall. 



coral (c, Fig. 352), which had appeared in the Ordovician; the honey- 

 comb coral (a); the organ-pipe coral (6); and the cup coral (e). A 

 most peculiar simple coral was quadrangular, and its top provided 

 with a cover (operculum) of four triangular plates hinged to the four 

 sides of the cup's margin. When closed they formed a pyramid over 

 the cup (d, Fig. 352, only two opercular plates shown). This was a 

 protective device unknown among modern corals. With their 



