CCELENTERATA : ACTINOZOA. I 5 I 



cannot extend above a certain level. Finally, the summit of 

 the reef is formed by an aggregation of less massive corals, such 

 as Madreporidcz, Milleporidce, and Gorgonidtz. 



DISTRIBUTION OF ACTINOZOA IN TIME. With the single 

 exception of the Mollusca, no division of the animal kingdom 

 contributes such important and numerous indications of its 

 past existence as the Actinozoa. 



In the Palaeozoic rocks the majority of corals belong to 

 the division Rugosa, these seeming to have filled the place now 

 taken by the sclerodermic Zoantharia. Until quite recently 

 it was believed that all the Rugosa were Palaeozoic, with the 

 exception of the genus Holocystis, represented in the Cretaceous 

 period (Upper Greensand) by the single species H. elegans. 

 Recent researches, however, have brought to light the existence 

 in our present seas of at least two genera (Haplophyllia and 

 Guynia) which belong to the Rugose family of the Cyathax- 

 onidiz; and certain Tertiary Rugose Corals have also been 

 described (Martin Duncan). As the Rugosa are in no funda- 

 mental structural character to be distinguished from the 

 Zoantharia Sclerodermata, save in the number of their septa, 

 there would thus seem to be no good ground for maintaining 

 that there is any essential difference between the Palaeozoic 

 corals and those of more modern times. 



The Cyathophyllidce. and Cystiphyllida are exclusively Palaso- 

 zoic ; the Cyathaxonida are Palaeozoic, but are represented by 

 two living genera ; and the Stauridcz are represented in the 

 Silurian Rocks (Stauria\ Devonian (Metriophyllurn), Permian 

 (Polycalia), and in Tertiary deposits (Conosmilia). 



The Zoantharia Sclerodermata, though attaining their maxi- 

 mum at the present day, nevertheless are well represented 

 in past time, beginning in the Silurian period. One subdi- 

 vision of this group, the Tubulosa, is entirely confined to the 

 Palaeozoic Rocks, and another, the Tabulata, is chiefly Palaeo- 

 zoic. The Perforata and Aporosa, on the other hand, are more 

 abundant in the Mesozoic and Kainozoic Epochs. 



The Zoantharia Sclerobasica are hardly known as fossils, but 

 the Miocene deposits of Piedmont (Middle Tertiary) have 

 yielded a species of Antipathes. 



The Zoantharia Malacodermata, from the soft nature of their 

 bodies, are obviously incapable of leaving any traces of their 

 existence ; though we are by no means therefore justified in 

 asserting that they did not exist in past geological epochs. 



The Alcyonaria are very doubtfully represented in rocks 

 older than the Chalk; the Lower Silurian fossil called Pro- 

 tovirgularia being more probably referable to the Hydrozoa. 



