6o 



THE CHAIN OF LIFE. 



one hand, and to a higher group, the Alcyonids, on the other, 

 both of which prefer four and eight to six, or have had these 

 numbers chosen for them. In the Mesozoic period the tabulate 

 and rugose corals were replaced by others, the porous and solid 



siiiiM 





■ '■' ■ynjir 



Fig, 4t«. — Zaphrentis frolijica (Billings). Devonian. 



corals of the modern seas ; but, in so far as we know, the 

 animals producing these, though differing in some details, were 

 neither more nor less elevated than their predecessors, and 

 they took up precisely the same role as reef builders in the sea, 



Fig. 45. — Rugose Corals. 

 a. Za/Arenii's Mi'nas {Dn.), a.nd d, Cyathophyllum Billiiigsiijixi.). Carboniferous. 



though with probably more tendency to the accumulation of 

 great masses of coral limestone in particular spots. 



Leaving the corals, we may turn to the sea- stars and sea- 



!? 



