ACTINOZOA. 243 



I 





with 



than the Jurassic. 



9. History of Rugosa. — 



All Rugosa are confined to the Paleozoic epoch, 

 with the exception of the genus Holocystis, which 

 is peculiar to the Lower Greensand, where it is 

 represented by a single species, H. elegans. 



The Rugosa first appear in the Lower Silurian. 

 They are especially abundant in the Upper Silu- 

 rian, Devonian, and Carboniferous deposits. In 

 the Permian rocks they are represented by only 



one generic form. 



4. History of Alcyonaria. 



Few genera of Alcyonaria have hitherto been 

 found in a fossil condition, and scarcely three of 

 these are wholly extinct. The existence of this 

 order during the Paleozoic epoch must be regarded 

 as doubtful, though the genus Protovirgularia 

 has been constituted for the reception of a Silurian 

 fossil, supposed to belong to the family Pennatu- 

 lidce. The genus Isis, also, has been stated to occur 

 in some of the Paleozoic formations. With these 

 exceptions no Alcyonaria have been found in 

 rocks more ancient than the Chalk. The family 

 Alcyonidce is without an extinct representative. 



5. Silurian Corals. — 



The Silurian Corals consist chiefly of Rugosa 

 and Tabulata. The Aporosa are represented by 

 the genus Palceocyclus ; the Perforata by Protarcea. 

 Doubtful indications of Tubulosa and Alcyonaria 

 also occur. At least nine families of Corals first 

 make their appearance in this period, and one, 



R 2 



