THE AGE OF INVERTEBRATES OF THE SEA. 59 



rugose corals closely resemble those of our modern reefs ; but 

 they differ in their' details of structure, and only a very few 



Fir. 43.— Tabulate Corals. 

 a, Halisites, and b, Favosites. Upper Silurian. 



modern forms from the deep sea are regarded as actual modern 

 representatives.^ One curious point of difference is that their 



Fig. 44.— Rugose Coral {Heliophyllum Halli). Devonian. 



radiating laminae begin with four, and increase by multiples of 

 that number, while in modern corals the numbers are six and 

 multiples of six ; a change of mathematical relation not easily 

 accounted for, and which assimilates them to hydroids on the 

 1 Haplophyllia, Guynia, Duncania, of Pourtales. 



