188 FOSSIL ACTINOZOA. 



in Oolitic times both in Western Europe and in England. 

 In the earlier portion of the Tertiary period, again, vast coral- 

 reefs were formed in Central and Southern Europe, in Egypt, 

 Syria, and Arabia, and in parts of India. 



As to the distribution in time of the Zoantharia sderoder- 

 mata, it is difficult to speak with precision, as much doubt 

 obtains as to the true systematic position of many ancient 

 forms often referred to this group. It may be certainly 

 affirmed, however, that the group attained no strikingly pre- 

 dominant position during the whole of the Palaeozoic epoch ; 

 that it underwent a great development in the Secondary 

 and Tertiary periods ; and that it has, perhaps, reached its 

 maximum at the present day. 



The Zoantharia sderodermata were divided by Milne- 

 Edwards and Haime into the four sections of the Aporosa, 

 Perforata, Tabulata, and Tubulosa, of which the two first 

 are large and natural divisions, while the two latter are of 

 doubtful affinities and uncertain value. We shall, however, 

 briefly consider the characters, geological distribution, and 

 leading types of each of these sections. 



I. APOROSA. 



The Aporose Zoantharia possess a corallum composed of 

 more or less compact calcareous tissue," the " theca " or wall 

 surrounding the visceral chamber being complete, and rarely 

 perforated ~by apertures or pores. The septa are well devel- 

 oped, and usually constitute complete lamellae ; and though 

 dissepiments or synaptieulae are present, tabulae very rarely 

 exist. 



Taken as a whole, the Aporosa are an essentially Sec- 

 ondary and Tertiary group, being represented during both 

 these periods by an immense variety of types. In the 

 Palaeozoic period, there is still some doubt as to the precise 

 position and structure of some of the corals which have at 

 one time or another been regarded as ancient forms of the 

 Aporosa. The Silurian genus Palceocyclus, usually referred 

 to the Fungidce, appears to belong rather to the Eugosa ; but 

 it is possible that the genus Columnaria (Favistella), from the 



