THE SILURIAN PERIOD. 407 



tance in the Silurian. They show advances in the preponderance of 

 elevated spires, in increased variety of form, and often in their large 

 sizes; but forms coiled in low spires, or in a plane, were still well repre- 

 sented (Fig. 187, xx, yy, zz). The pteropods were few, but the peculiar 

 form Conularia was an interesting representative. 



The pelecypods (Fig. 187, ww) were not as well represented in the 

 Mid-Silurian beds as they were in the Ordovician, perhaps because 

 the calcareous bottoms were less congenial to them. Some of the 

 forms, however, were quite robust and displayed notable advances 

 over previous forms. 



Coral development and the inauguration of reef-building. — The 

 prominence gained by the corals in suitable situations is one of the 

 notable features of the Silurian fauna. In the Ordovician period the 

 simple forms predominated over the compound. The ratio was now 

 reversed, and the latter became very abundant. Among the notable 

 types was the unique chain coral Holy sites (Fig. 188, c), which was 

 occasionally present in the Ordovician, but now became abundant; 

 the honeycomb coral, Favosites (Fig. 188, a), accompanied by a group 

 of allied genera; the organ-pipe coral, Syringopora (Fig. 188, b), and 

 the cup coral, Zaphrentis (Fig. 188, e). A most peculiar coral of the 

 simple class was Goniophyllum (Fig. 188, d), which instead of assuming 

 the usual conical cup form took on a quadrangular shape and was pro- 

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

 the four sides of the cup's margin, so that when closed they formed 

 a pyramid over the cup (Fig. 188, d; only two of the opercular plates 

 shown). This was a protective device quite unknown among modern 

 corals. 



With their increase in abundance, the corals acquired the habit 

 of associating themselves together, which resulted in the accumu- 

 lation of coralline debris and the formation of reefs which, as they 

 grew, rose above the common sea-bottom and secured the benefits 

 of purer water and more active circulation. The known coral reefs 

 appear to have stood some distance from shore, and to have been 

 of the barrier type. Other life in great abundance and variety gathered 

 upon or about these reefs, and they became rich depositories of the 

 ancient faunas. 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. 



