176 



CHAPTEE X. 



FOSSIL ACTINOZOA. 



OF the living groups of the Actinozoa (see Table, p. 153), the 

 Ctenophora and the Sea-anemones (Zoantharia malacodermata), 

 from their absence of hard parts, are unknown in a fossil 

 condition. The remaining groups viz., the Zoantharia 

 sclerobasica, Zoantharia sclerodermata, Alcyonaria, and Rugosa 

 secrete a hard skeleton, which is known by the general 

 name of the " coral " or " corallum." All these groups, 

 therefore, are known as fossils ; but they are of very un- 

 equal importance. The Zoantharia sclerobasica are known 

 by very few fossil representatives, and require to be little 

 more than mentioned. The Alcyonaria, also, with the excep- 

 tion of the remarkable group of the Heliolitidce, are of little 

 geological importance. The Zoantharia sclerodermata and 

 Rugosa, on the other hand, have left very numerous and 

 interesting traces of their former existence the latter being 

 almost altogether extinct, and both will require to be 

 noticed at some length. Eegarded as a whole, the class 

 of the Actinozoa appears, so far as we yet know, to have 

 commenced its' existence in the Lower Silurian period, and 

 to have attained its maximum of development at the pres- 

 ent day. 



ORDER I. ZOANTHARIA. 



Tentacles simple, rounded ; soft parts in multiples of five or six. 

 Sub-opder 1. Zoantharia malacodermata. Ex. Sea-anemone. 

 Sub-order 2. Zoantharia sclerobasica. Ex. Antipathes. 

 Sub-order 3. Zoantharia sclerodermata. Ex. Reef-building Corals. 



